Why is 16. d5 a blunder in this position? In this position I'm White and I have just played 16. d5. Stockfish labels the move as a blunder. I do understand I had better moves than this, and it's not the move I would play if I were to get this position again, but why is it a blunder? It doesn't seem that bad. My reasoning was that, having the bishop pair and better development (two minor pieces playing versus one), and Black having a somewhat vulnerable king, I could benefit from opening some lines with the pawn-break.
rn4k1/pbp1qr2/1p1pp2p/3P1pp1/2P5/2PBP1B1/P1Q2PPP/R4RK1 b - - 0 16
The move 16. d5 is not necessarily a blunder. Although it is not good for White, it is not losing. Lichess and other chess engines sometimes see positions differently from how humans would percieve them.

That being said, analysis on chess.com deems it a a mistake. which is much more reasonable. The reason is that, in the long-term, the d5 pawn mught be lost, giving Black a +1 advantage, in material worth, at the least in some lines.  Even if White retains a pawn, the doubled pawns on the queenside will be a disadvantage that Black can use, or there will be some other disadvantage Black can create for White. 

All in all, White will have some sort of positional weakness no matter the outcome.

Here is a computer generated line that shows how Black gets the upper hand in the long-term with an extra pawn. I do not claim that this is the 100% best play, just that it shows one of the many general outcomes where Black has an advantage.

[FEN "rn4k1/pbp1qr2/1p1pp2p/5pp1/2PP4/2PBP1B1/P1Q2PPP/R4RK1 w - - 0 1"]

1. d5 Nd7 2. Rae1 Raf8 3. f4 Nc5 4. e4 fxe4 5. Bxe4 gxf4 6. dxe6 Bxe4 7. Rxe4  Nxe4 8. exf7+ Rxf7 9. Rxf4 Rxf4 10. Bxf4


Note that although f4 is technically an inaccuracy, it is perhaps best by human play.

Here is another line that shows how Black can get an advantage in other ways as a result of White’s mistake.

[FEN "rn4k1/pbp1qr2/1p1pp2p/5pp1/2PP4/2PBP1B1/P1Q2PPP/R4RK1 w - - 0 1"]

1. d5 Nd7 2. Rae1 Raf8 3. f3 Ba6 4. Be2 e5 5. e4 f4 6. Bf2