How is 26.Bc1 a blunder ?

I don't think it's so much that Bc1 does something bad but rather than there was a much better opportunity missed with 26.Rf1.   The idea behind 26.Rf1 is that it sets up a discovered attack - White is threatening to move the knight at f3 and the rook at f1 will then have a discovered attack on the Black queen.

What makes this threat more complex and dangerous is that often Black would just cancel out the threat by moving his queen off of the f-file but in this position the Black queen is short of safe squares.  The engine suggests Black plays 26...Kc7 so that White's Ne5 is no longer a check but the threat of the discovered attack on the queen remains.  I think the engine's line is complicated but it claims that 26.Rf1 allows White to draw.

Thank's for the feed back, I get it now. But on the basic analysis it said it was blunder. This was a 10 minute blitz game. 

Probably because 26.Bc1 loses and 26.Rf1 draws.  Though I don't think the draw is straightforward.  Personally I find this position complicated and I think I would have missed Rf1.

I don't know that rf1 draws, my stockfish thinks the position is still bad at depth 27, but bc1 is met by qf8 and then what?  At least rf1 poses some problems and gets the rooks linked.  Breaking the communication between the rooks for nothing just makes the position worse.  

The time controls have nothing to do with the evaluation.

I agree with ArtNJ, Black is still quite a bit better after 26.Rf1, but it's still a better move that 26.Bc1 which only has value if Black hangs his queen. But it's useful to remember that terms like "blunder," "brilliant move," "Excellent move," etc. are English phrases put in by programmers to reflect the difference between the numerical evaluation of the computer's first choice and your move. 

Only saying I may have made a different move, given more time .... I alway's presumed excellent was better than brilliant, but apparently not on here. Must be the programmer's choice. I didn't play 26.c1 in hope's my opponent would hang his/her Queen, but to gain a tempo in the game. 

The issue with Bc1 is that you are not in fact gaining a tempo, but losing time. So black is forced to move his queen eg.Qf8, this is not such a big problem for him. You meanwhile must move your bishop again so that the Ra1 can participate. What is the best square for the Bc1? Probably Bb2 where it just came from. On the other hand Rf1 creates an immediate threat as @ThinkFreely points out, and this move does gain a real tempo. 

I think you should be looking more at move 24.  Up to that point you had a much better position than Black.  And I get frustrated to with people invading with their Queens but I've heard many a masters say you usually need 3 pieces to create a proper attack.  After he played 23...f4?? you had the simple 24.Ne4 trapping his Queen and effectively winning the game.  I understand the temptation to "chase" the enemy Queen out but after his 23...f4 you have nothing to fear. 

Back to 26. Bc1 after the Queen retreats you have just lost a piece 26. Bc1 just attacks the knight on h6 but the Queen is able to defend and isn't really doing much on that square other than being misplaced and blocking your Rook.

I know it doesn't gain, an absolute real temp now having even looking at myself now. But at the time I thought it did, and the best move I could come up with, at that specific part of the game.

I won the game anyway on time control. On a slower time control however, I may or may not of seen 24.Ne4. 

Yes it's annoying when it calls missing the gain of material "a blunder" and it shouldn't do so because sometimes, what it thinks is a blunder is a stronger move than the computer's move. But in this case Rf1 is strong and you might play the Bc1 later.

Isn't it Rapid?

10 mins is blitz. 15 is rapid