Assessing an early c4 against white's Stonewall structure I made a horrible decision as black in a game, and I do not understand why it's incorrect.

[fen "rn2kb1r/pp2pppp/1q3n2/2pp4/3P1P2/2PQPN2/PP4PP/RNB1K2R b - - 0 1"]


I played c4, Stockfish considers the position to be + 0.4 after the move. My reasoning: I forever block white's bad bishop behind his blocked pawns, while keeping my good bishop. Since I gain a tempo on the queen, I can move my queen right away to be able to answer the break b3 with b5. 

I am also thinking that my position is much better, as white has a hole on e4 and I don't have such a weakness, so I thought I'd capitalize on my advantage by closing up the position. 

Stockfish considers this move to be a really poor move, e6 leading to an evaluation of -0.7. 

I've had this several time, and it's unclear to me why my move is bad, even though it gives me an inferior position from a superior one.

Is it always bad to close up the position this way, or is it specific to the position ? Why is it bad, because I forever lose my main pawn break ? Or because my "good bishop" has no target and is therefore not really good ?
Good question! The positional priorities in this position do not really lie in whose bishop has more prospects, but rather in the emerging pawn structure, potential pawn breaks, and either side's ability to create targets and holes in opponent's camp. 

In short, c4 here is an extremely committal move which should only be played if it can be backed by very strong concrete ideas, which do not exist in this position. Why does the fact that it's committal makes it into a poor choice? It's simply because it resolves white's strategy into something immediate and concrete, namely: to undermine black's pawn chain and create a weak pawn out of c4, and to this end, the almost immediate e4 pawn break is perfectly playable for white without any drawbacks and black has nearly no means of preventing it. So in hindsight, moves such as c4 and similar ones in analogous positions, are only good if you are sure you can stop your opponent's relevant pawn breaks. Otherwise you're creating overly advanced pawns that will eventually turn into weaknesses that you cannot easily parry. 
Remember, a pawn is considered to be weak if (all 3 must be met simultaneously):


it cannot move
it is isolated (not defensible by pawns)
it can be attacked. 


To summarise: with the unavoidable e4 by white and a4 in reply to b5, the c4 pawn is bound to become a weakness as it meets all 3 said conditions. 
Gained tempi and temporarily blocked out bishops are secondary in such positions, unless you have an actual threat following your pawn push (such as, an imminent knight jump to d3!)

More concretely, let's take a look at a few possible continuations as illustrated in the following diagrams:

Left: Early e4 by white  |  mid: early b5 attempt  |  right: e4 blockade attempt






Left: Immediately undermining c4's defense after Qe2, Nbd2, if black takes, c4 is dropping soon and both knights will be well established centrally. And if black tries to sit still, e4-e5 is a threat, leaving black's kingside heavily undefended and white's pawn push is quite straightforward, black will lack space and the ability to create a counter-attack.
Mid: Black prepares for the immediate b5 (with Qc7) in order to reinforce c4 which leads to: Qe2, Qc7, Nbd2, e6, e4, b5, f5! and soon followed by a4. Notice how white's perfectly undermining the entire pawn backbone (e6, d5, b5) of c4.
Right: Black simply tries to stop e4 by blockading it with a knight, but since it cannot really be followed up by f5 (as shown in the diagram), it will easily be challenged with Nbd2 after which black either has to take on d2 which revives e4 again, or lets white take on e4 leaving a ton of light square weaknesses behind along with a permanent knight post on e5. Note that in this structure, black has no prospect for ever landing a knight on d3, leaving the light square control that the overly advanced pawns have secured virtually unusable. Finally, notice that in neither of these cases, black's left with any form of initiative or targets in white's structure to allow for any counter-attack. 


Lastly, by not playing c4, you can still entertain a timely resolution of the central pawns in order to either simplify or create weak points in white's centre. And to connect with our discussions, by holding still with the pawn on c5 you are actually stopping white's e4 push as after, say, e6, Nbd2, Be7, e4 you can simply take both on e4 and d4 not allowing white any central control (unlike the case of c4 where white always had clear spatial advantage), and your structure is devoid of weaknesses. If anything, white will be the only side with potential dark square weaknesses. And if white recaptures on d4 with the pawn, then it's a comfortable IQP structure with easy play for black.