Blacks last move was
10 d5...
An example:
The white pawn cant see the black pawn. And white cannot see the d6 square change colour to indicate a possible en passant becasue his bishop covers d6 already - so it is already visible.
In this instance, how is white supposed to know he can capture on d6 en passant?
he isint (:
how is white supposed to know he can capture on d6 en passant?
Testing! the d4 pawn gives us the key! the d5 square suddenly darkened, who has occupied it? I don't know, now try to take the step, if it works, the square was occupied by a pawn, if it does not work, it is occupied by a piece. As simple as that.
but in this position, the d5 square doesn't darken
how is white supposed to know he can capture on d6 en passant?
Testing! the d4 pawn gives us the key! the d5 square suddenly darkened, who has occupied it? I don't know, now try to take the step, if it works, the square was occupied by a pawn, if it does not work, it is occupied by a piece. As simple as that.
Indeed! I realised this after I posted!
My own thoughts are personally that in FOG, the ability of pawn to "sneak" past their opponents is one of the strongest elements of the variant. Pawns are far mor powerful in FOG. And sure! Why shouldn't they be able to sidestep en passant 
but in this position, the d5 square doesn't darken
Although this position is invented, if now Black plays d5 there is no way to know if he played d5, correct, but now intuition and imagination come into play, and I try to eat in step, if it works, it is that they moved the pawn, yes It does not work, we should think that they made black
how is white supposed to know he can capture on d6 en passant?
Testing! the d4 pawn gives us the key! the d5 square suddenly darkened, who has occupied it? I don't know, now try to take the step, if it works, the square was occupied by a pawn, if it does not work, it is occupied by a piece. As simple as that.
Indeed! I realised this after I posted!
Something interesting can be seen here https://en.wikipedia.org/wiki/Dark_chess
I always thought it was asking quite a lot to notice the color change in the case of en passant, but I've become used to it and come to accept it as part of the game.
Intrepidberk has changed my mind. I think it's a flaw in the design of the game if there's no indication of the possibility of capture.
In the position posted by noafavelo, imagine a knight on e4. There would be nothing to signal that a pawn had moved to d5 and attacked the knight. Normally there would be, but in this case the information would be hidden because of the bishop's sight. In effect, White is penalized for having good board control.
The rules shown on the page for Dark Chess make perfect sense. The pawn that is available for capture via en passant should be visible. It's really a simple change, and I see no down-side to it.
please dont mistake me - I like the way FOG plays. I just see en passant as being one of those things that forces the player to be alert to its possibility. Naturally, I also like the idea that pieces can be taken wthout forewarning - thats the appeal of the game. But I was also cognizant of the OP's comment that in certain circumstances, it is impossible to know that an en passant capture is available.
I just played a game where I was a bit confused. It seems I had the opportunity to play en passant but the indication for this was only that the square that my pawn could move to capture went from dark to empty. It would have been much clearer if I had been able to see the pawn too.
If I had another piece watching the en passant square I would not have any indication that I could play en passant at all. That seems to be a bug. Normally you can see all the pieces that you can capture.