You must remember Thomas Gainsborough’s Blue Boy. It’s the painting that gets stolen from a rich man’s house in the 1929 Laurel and Hardy short Wrong Again. Trying to claim the reward, Stan and Ollie fetch a racehorse called Blue Boy instead and attempt to “leave it on the piano” as they were asked. If only the National Gallery screened this in their tiny exhibition to mark The Blue Boy’s return to Britain for the first time in a century, we might get a sense of what it once meant. In 1921 railway magnate Henry E Huntingdon bought this 1770 painting for a world record price, making it as famous in 1920s America as Banksy is now. Perhaps The Blue Boy even inspired pop art, for it injected glamour into jazz age Hollywood. But that was then. This glittering dazzle of blue and silver with a bonny face on top has come home far too late. Shown with two other Gainsboroughs and two portraits by his inspiration, Anthony van Dyck, it embodies a vision of English art that’s as hokey as a Disney cartoon of a fox hunt. It makes you wonder if as a nation, when it comes to art and the soul, we have something missing. I thought I loved Gainsborough’s virtuoso brushwork, like a bravura bit of Mozart or Haydn in paint, pleasing itself in molten chromatic fripperies. But staring too long at The Blue Boy is a good way to get fed up with his talent. This painting of an entitled, arrogant youth in a fancy dress costume has a country-house worth of technical magic but no soul. It surely appealed to people a century ago – including Americans who still emulated the English upper class – as a quintessentially English icon of snobbish disdain and style. Today it might stick in our throats for the same reason. But that is not, exactly, what is wrong with it. The craft of this show-off painting scintillates. You enter the small room where it is on display and are delighted by that burst of silvered azure. Whoever this adolescent was – the identity of the Blue Boy has never been established – Gainsborough turned him into precious artwork. He got him to dress in an archaic costume inspired by the cavaliers Van Dyck painted in the 1600s on the eve of the English civil war. Van Dyck, from Antwerp, created a definitive swaggering look for high-style English aristocratic portraiture. In Gainsborough’s day, when portraitists were vying for the custom of an elite enriched by the agricultural revolution, global trade and slavery, the Van Dyck manner was hot. So, in this tour de force of oil painting, Gainsborough takes on Van Dyck and out-primps him. Van Dyck’s portrait of Lord John Stuart and his Brother, Lord Bernard Stuart, painted about 1638, is hung for comparison. These two youths with expressions of ice-cold hauteur become a bit more sympathetic when you realise they would both shortly be killed in the civil war, probably while brushing their long hair. Lord Bernard Stuart wears a silver and blue cloak, blue pantaloons and fawn boots. He poses with one knee raised, hand on hip, looking down on us over his shoulder. Gainsborough’s youth wears a version of this very garb yet it is painted much more playfully. Van Dyck’s brushwork is meticulous. Gainsborough’s is rollicking. He lets loose glancing dazzles and dips of light and cool in every crease, riffs on each frolic of lace like a drunken flautist. He has great fun, riffing on ruffs. If that was enough to make a portrait great, this would be one of the greatest. However, it is not. The painting’s most brilliant visual coup is also the beginning of what it lacks. Gainsborough sets off the electric storm of blueness against a rainy English sky. As the Victorian critic Ruskin noted, Gainsborough’s weather is the most British thing about him. This sky is brown and brackish, laden with chilly moisture, a sublime bass note to contrast with the piping flute. It is a romantic landscape – one of the first. Yet Gainsborough does not use it romantically. He uses it for pure visual effect: the morose setting exaggerates the figure’s brightness. Instead of a solitary walker lost in landscape we get an overdressed cocksure kid looking right through us. Gainsborough sacrifices emotion for effect. The Blue Boy’s face doesn’t betray the least hint of introspection. In reality, the youth’s aristocratic demeanour may be a put-on. One possible candidate for the model is Gainsborough’s nephew and apprentice Gainsborough Dupont. That makes sense if, as I suspect, The Blue Boy is not a real portrait so much as a display piece Gainsborough kept in his studio to razzle-dazzle potential customers. It was painted when he had a portrait business in Bath. Gentlefolk staying at this spa town for the season could pose for him between playing cards at the Assembly Rooms and marrying off their daughters. Gainsborough called it “pickpocketing the rich”. And yes, he was happy to pickpocket people whose money came from slave plantations. To me The Blue Boy looks like his brilliant advert – today it might be a video he played for the client so they could see how he might transform them. That would explain why it has so much style and so little soul. Gainsborough could be so much better than this. And the National Gallery accidentally shows that by including one of his real masterpieces, Elizabeth and Mary Linley, which is a risky move if you want people to look at anything else in the room. Mary Linley returns your gaze with true feeling and intelligence while her sister looks into the landscape with Romantic feeling. This is how Gainsborough could paint people he really cared about – who usually tended to be women, or fellow creative workers. The musical Linleys were both. These paintings are keepers but The Blue Boy had me fidgeting after five minutes. It is as boring as it is impressive. It seems the art market a century ago was as silly as it is today. Gainsborough’s Blue Boy is at the National Gallery, London, from 25 January to 15 May.