Darcy really wanted to upset Lizzy here.I think he was genuinely flirting with her.

Scotch airs were very popular during our era and throughout the 18th century, so ineffect they were talking about very popular music, not anything particuarly highbrow .And I think both Darcy and Lizzy liked them ;-)

Look at this small passage from The innocent Diversion: Music in the Life and Writings of JAne Austen by Patrick Piggott about Scottish airs and Darcy's request:

This was the period when "Scotch Songs" were the rage throughout all Europe. The collections brought out by several Edniburgh publishers, often with accompaniments by the greatest composers of the day , inlcuding Haydn,Pleyel and later Beethoven, had an immense sale.

The demand soon exceeded supply and the compostion of fake Scottish melodies became a luctrative trade for impecunious British composers.

However the music Miss Bingley played might not have been one of these pesudo-Scottish songs but a piece of genuine Scottish dance music , since it prompted Mr Darcy to ask Elizabeth if it did not make her wish " to seize the opportunity of dancing a reel"
Page 54

I think this shows that Darcy is not as stuffy as we ( and Elizabeth ) might have first thoguht : he is a relatively young man after all.

Elizabeth thinks the worst of him, that he is trying to trap her into admitting a liking for a very popular style of music, so that the can look down at her. She actuallly misjudges him,however,IMHO. He is very gracious when she turns him down, and I think may have genuinely liked "Scotch airs":

"Oh!" said she, "I heard you before, but I could not immediately determine what to say in reply. You wanted me, I know, to say 'Yes,' that you might have the pleasure of despising my taste; but I always delight in overthrowing those kind of schemes, and cheating a person of their premeditated contempt. I have, therefore, made up my mind to tell you, that I do not want to dance a reel at all -- and now despise me if you dare."

"Indeed I do not dare."

Elizabeth, having rather expected to affront him, was amazed at his gallantry; but there was a mixture of sweetness and archness in her manner which made it difficult for her to affront anybody, and Darcy had never been so bewitched by any woman as he was by her. He really believed that, were it not for the inferiority of her connexions, he should be in some danger.

I think had she said "yes", there would have been a reel danced in the Netherfield drawing room that night ;-)