There are game jams about all kinds of things. The subject doesn't really matter so much as the constraints introduced and the creativity that flows from game designers working to overcome them together. So why not a game jam about watermelons?
I haven't seen Blade in a while, but one of my primary memories of the film is when Wesley Snipes whips out his multi-bladed chakram and wipes out like, 5000 vampires at once. Blood everywhere! Sunglasses! Trenchcoat!