William Blake: The art of a 'lunatic'?

If beside the stupid and mad-brained political project of their rulers, the sane part of the people of England required fresh proof of the alarming increase of the effects of insanity, they will be too well convinced from its having lately spread into the hitherto sober region of art. But when the ebullitions of a distempered brain are mistaken for the sallies of genius..." – well, then you know that you're in the middle of an art critic, furiously boiling over.

The unfortunate facts are these. In May 1809, William Blake opened a small exhibition of his work on the first floor of his brother's sock-shop in Golden Square in Soho. Very few people came. It is not clear if anything was sold, but Blake kept the show up for more than a year, indicating a reluctance to accept defeat.

There was one, utterly contemptuous review (as above). And six of the sixteen pictures subsequently disappeared, including much the largest work Blake ever made. This year it's the 200th anniversary of an unmitigated failure, which pushed the artist into the withdrawn state of his later years, and set him on the way to almost a century of oblivion.

The building itself has gone, so there's no question of a restaging on site. But Tate Britain is remembering this important non-event in the history of British art with a one-room display, which includes all the surviving works from "Poetical and Historical Inventions" (as Blake titled the show), alongside a few other pictures also exhibited in London that year (by Turner, for instance) to give a sense of contrast.

It also remembers the six lost works. It represents them as visible absences, with white oblongs marked out on the blue-grey walls. Most of them are small, as Blake's images usually are. But one of them is suddenly enormous. Fourteen feet by 10, The Ancient Britons had epic proportions, and showed three over-life-sized figures taken from a version of old British mythology.

Blake names them "the strongest man, the beautifullest man, and the ugliest man". They were the last of the Britons, after having been defeated by King Arthur (who Blake was against). They were naked, based on antique statues that Blake identifies, with healthy reddish complexions, and appear on the battlefield with a bard lamenting among the dying.

I found myself standing and staring into that big blank rectangle, wondering what exactly filled it. Blake's description is quite detailed, but with no studies or sketches you can only guess how The Ancient Britons would actually have looked. Given the desperate state of other pictures that Blake painted in what he called "fresco" – Satan Calling Up His Legions hanging here is almost black – it may well have decomposed entirely. If not, there's a gigantic revelation waiting somewhere. I can also imagine some modern Blakean attempting an imaginative reconstruction: perhaps not such a good idea.

But what we know of this picture holds a message, at least. Blake had not yet become an inward-turned visionary. He was working here on a public scale. He was an artist with public ambitions. He announced that his images were "seen in my visions", yes, but he offered them for public benefit, as other works here demonstrate.

The 1809 show contained possibly Blake's two strangest pieces, a pair of mythic and patriotic pictures. The Spiritual Form of Nelson Guiding Leviathan and The Spiritual Form of Pitt Guiding Behemoth are an apotheosis of the military and civic leaders of the nation in the recent French war. Nelson is Apollo. Pitt is Christ. In other hands, it would be just an extremely complimentary metaphor. In Blake, it seems to represent a supernatural reality, as if the battles were being fought on the astral plane.

That's a big difference between Blake and other contemporary artists, like Henry Fuseli and James Barry, who were his friends and admirers, and who were certainly considered eccentric too, but managed to rise in the art establishment, with high positions in the Royal Academy. Blake hadn't at this point become a total outsider. His ideal of "the grand style", meaning an epic allegory, was an official academic ideal. But his claim to be the only artist to have achieved it, to have revived the art of Michelangelo and Durer, and to have been shown how to do it in visions, obviously sounded cranky.

The Golden Square exhibition was accompanied with Blake's Descriptive Catalogue. It contains these claims, and an account of each picture, along with an explicit statement of Blake's artistic doctrines. Here are his violent attacks on oil painting, on Titian, Rubens and Rembrandt, with their blurring and blotting. Here is his noble declaration against vagueness. "The great and golden rule of art, as well as life, is this: That the more distinct, sharp and wirey the bounding line, the more perfect work of art." Tate has issued this key document in a nice new edition.

In addition to his own manifesto, Blake's one-man show yielded another text: that solitary and rude review written by Robert Hunt. Hunt wasn't a political reactionary. He wrote for a radical publication, The Examiner. Not that this makes his rejection of Blake surprising. Lefties are often artistically conservative. Still, it was the only notice Blake got in his lifetime, it's quite long, and it could be a valuable document. It could tell us about Blake's art.

But no. It's all so timelessly familiar. Hunt carries on like any critic on his high horse. He spends most of his review mocking the Descriptive Catalogue. Admittedly, with its flagrant arrogance and pretentiousness, it makes an irresistible target.

Hunt has no trouble turning out lively sallies that might come from any artistic Schimpf-Lexicon: "an unfortunate lunatic", "the poor man fancies himself a great master", "deformity and nonsense", "a farrago of nonsense, unintelligibleness, and egregious vanity, the wild effusions of a distempered brain." This vigorous abuse was no doubt amusing for the critic and his readers, and is now amusing again for us, in all its confident mistakenness. But the problem about Hunt's jaunty phrasemaking is not that it now appears wrong. It's just a waste of time. It tells us the critic's opinions. About Blake's art it tells us nothing.

It would be very good to know how Blake's art looked to his contemporaries. We gather it could look weird and wonky and worse. Here a critic should be of assistance. But saying "badly drawn" as Hunt does twice is hardly enlightening. His only specific term of criticism is caricature, again twice – "an attempt at sober character by caricature representation", "this picture is a complete caricature" (The Ancient Britons). It isn't very specific either, since it could mean inadvertently comic or misproportioned or stiffly drawn or grotesque, but it's a start.

Beyond that, we haven't an inkling whether Hunt was able notice the things about Blake that are so obvious to us now. His visual world of archetypes, with its symmetrical compositions, its strongly flat designs, its figures turned into geometrical shapes, its use of a cartoon language of zooms and whooshes: could Hunt see these qualities at all? Exactly how did he see them?

This show and this review, 200 years ago, hold a lesson for any critic now. Your artist may turn out to be great genius (it's always possible). Even if not, somebody may be interested. Either way, later viewers will thank you, not for your insults, but your explanations. If your artist seems all wrong, then say precisely what seems wrong. However negative your view, be as clear as you can about what you see. Like artists, critics work under the gaze of posterity too. We should try to be a little helpful.

William Blake's 1809 Exhibition is at Tate Britain, London, SW1 until 4 October. 'Seen in My Visions: a Descriptive Catalogue of Pictures' is published by Tate, £12.99