The Blumenfeld countergambit is a variation of the Benoni. The original form of the Benoni (with c5 played before Nf6) is considered to give White too much. This is why c5 is only played after c4 and Nf6 (it's not so much about Nf6 being included; it's more about c4.) The variation without these moves included is called the Old Benoni. As far as I know the Blumenfeld is good if White accepts but the critical variation is to decline, but I don't play it. Someone else will give you a better answer.
Blumenfeld countergambit - Different from Benoni?
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Apr 17, 2022
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#4
The Blumenfeld specifically includes the move Nf3 from White, and typically comes from a Nimzo move order. The Blumenfeld is closer in line to the Benko/Volga Gambit than the Benoni.
The Blumenfeld is 1.d4 Nf6 2.c4 e6 3.Nf3 c5 4.d5 b5. The Blumenfeld cannot be played as a standalone defense. Black must have something else for 3.Nc3 and 3.g3.
The Benoni is 1.d4 Nf6 (1...c5?! Leaves the c4-square open for the White Knight to go Nf3-d2-c4 and a2-a4, attacking d6) c4 c5 3.d5 e6 4.Nc3 exd5 5.cxd5 d6 6.e4 g6 and now the major difference, and why most GMs play the Nimzo move order and only the Benoni against 3.Nf3 (no 4...b5 though) is because of 7.f4!. With a N on f3, that is not possible.
How is the blumenfeld countergambit different from the benoni? I see that the knight is on f6 prior to playing c5, but is that all.
Can anyone say if it is relatively sound compared to other gambits? To me it simply appears to be a benoni with the knight on f6?