Position from the Dutch Defence / Blackmar-Diemer gambit In the Dutch Defence, the following gambit isn't very popular anymore.

[FEN ""]
1. d4 f5 2. Nc3 d5 3. e4 dxe4 (3... fxe4 4. Qh5+)


The same position can arise from the Blackmar-Diemer gambit if black plays the rare 3... f5.

[FEN ""]
1. d4 d5 2. e4 dxe4 3. Nc3 f5


Scheerer, on pages 62-68 of his book the Blackmar-Diemer gambit, analyses 4.Bf4 and seems to like white's compensation.

What is the true evaluation of the position?

If it's good for white, why isn't the 3.e4 gambit against the Dutch Defence more popular?

If it's good for black, why isn't 3... f5 played more often to counter the Blackmar-Diemer gambit?
You raise a very interesting point - you're absolutely correct in saying that 3...f5 is not a common response to the Blackmar-Diemer gambit. However, the gambit can be reached in another way where Black doesn't have the option of playing this move:

[FEN ""]
1. d4 d5 2. Nc3 Nf6 3. e4 dxe4 4. f3


In the Dutch, Black can sidestep White by playing this move:

[FEN ""]
1. d4 f5 2. Nc3 Nf6


Your question as to why it isn't played more in the Dutch/more against the Blackmar-Diemer is a bit difficult to answer. Aside from the examples above where White or Black can sidestep this line, it can be reached in the main line of the Dutch against 2...d5 and the mainline of the Blackmar-Diemer with 2. e4. It should be better for either White or Black, so why don't we see it more often?

In my opinion, I think that White has sufficient compensation for the pawn due to the weak e5 square and the possibilities of playing f3, undermining Black's pawn structure. If we go with this assumption, then we're looking at the first scenario in the Dutch Defense. I believe that there are more common replies to 1...f5 such as 2. g3 and 2. Nf3 which tend to score better than the gambit line outlined in the question.

On the flip side, I noticed that Black actually scores better with this line in practice, so I'm rather quite torn. I believe you've hit upon something rather original, and I encourage you to do a bit of studying into the lines yourself and arrive at your own conclusion :) maybe you have found a new main line!