Refutation of the Blackmar-Diemer gambit? 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?
The simplest refutation is 2...e6, the french defense, one of the most stable openings for black. 
It is also fine to accept  the gambit  2... dxe4, but you have to know some specific traps....