yep, OED confirms the usage:
1892 Sporting Times 29 Oct. 1/2 Then I sallied forth with a careless air, And contented raspberry tart. 1959 I. & P. OPIE Lore & Lang. Schoolch. i. 9 Breaking wind was, at one time, by the process of rhyming slang, known as a ‘raspberry tart’.

and here I was all set to come up with some convoluted conflation of the word rasp (an unpleasant sound) + an ironic use of berries, as in "it's the ~".

-joe (Entia non sunt multiplicanda praeter necessitatem) occam