Is there an accepted refutation of the Blackmar-Diemer Gambit, 1. d4 d5 2. e4 or what is the strongest line for Black to prove that the gambit is unsound?
Not an expert, but I'd say the onus is on the gambiteer (White) to prove anything. W is delaying development and must waste time to capture the pawn later. Black has natural moves like Nf6 and Bf5 which both develop and delay W's capture of the pawn (and can possibly even hold on to it). W is just giving away some of the opening advantage. I don't think Black can expect more than this. W can also choose to give up a pawn altogether with f3, exf3, Nxf3 with in-adequate compensation.