Good photo but still too little to go on.
Can you wait until the buds break? Bear in mind that all commercial blueberries are hybrids of wild species - there are several with different leaf "aspect ratios", but apart from that there are many similarities, so it should be obvious if they are willow.
You might be the proud owner of a native bilberry patch.
Taste of the leaf might be a useful difference.
If you don't want to wait a bit then I think you'd have to go to a supernerd - try NatHist museum or agricultural research stations like East Malling...
Cheers.