Why was castling here a blunder?

Because 7.h3 8.Qg2 Rh2 wins the black queen.  So does...

7.h3 Qh5 8.g4 Qg6

Ah, I see, it does trap the queen. Thanks for the responses!

This is why chess engines can be deceiving.  Your move wasnt a blunder in the true sense.  A blunder is a move that loses the game, and your move doesnt lose, it just misses winning the queen.  

Be CAREFUL when using engines.  You were right in asking "why" here.

I see. I was confused there. Thank you!

When I was kid folks used !? = interesting ?! = dubious ?? = blunder ??! = oversight.
So for me it would be 7. 0-0 ??! and all them characters would tell me to look at the position. 

king defence is first priority people never see the trap

Yes, it was a blunder in the true sense!  A blunder is not a losing move.  It is a move that DRASTICALLY changes the assessment of the position in the negative direction for the player that made the move.

For example:  WKd6, WQf1, BKe8.  White to move.  Almost any move wins for White, and moving the Queen to any other square on the f-file from the 5th rank on back (i.e. Qf2, Qf3, Qf4, or Qf5) is mate in 2.

1.Qf8?? and 1.Qf7?? are both outright blunders because the Queen hangs and 1.Qf6?? would be an outright blunder because of Stalemate.  None of these moves "Lose" the game, but they are all outright egregious blunders as they flat out change the result of the game from Winning to Drawn (i.e. +49.63 [made up the number] to 0.00).

how did the engine tell you it was a blunder? it should have given you an eval, and whatever eval 0-0 is, its no blunder.

Excellent observation.

It does make sense, though, that a blunder can be defined as something that greatly reduces one's advantage.

@darkunorthodox88, I think I was still winning, but my numerical advantage (cp, apparently it's called) went down a lot.

First off, is White really "WINNING" or just "BETTER" after 7.O-O?

Secondly, +50 and +2 is a drastic change.  +50 usually implies that White is winning and it's virtually impossible to screw up.  +2 often means theoretically won, but there may still be work to be done, and a series of correct and possibly difficult moves must be found.

+50 to +2 is a DRASTIC change in the position, though White is "winning" in both cases.

you could have trapped the queen

I certainly understand what you are Thrillerfan are saying, and i can see the point.  But...Just my opinion...It is not a blunder in that the OP's move does not ruin his position.  But lets not get off track here.  We arent here to discuss the defintion of a blunder, but to help the OP understand "why"

Kinda like, how many more people will post "You could have trapped the queen" When that question has already been answered.

I feel y'all are arguing semantics now. Even if it wasn't called a blunder, I'd still take the queen if I'd seen that I could.

And that is what matters...the learning.

Yeah sure... that's gonna happen.

This is not a blunder, It's a overlook. Think about "slower checkmate": It's not a blunder(sometimes a mistake). 7.O-O isn't a blunder too, even though white has some work need todo and black has a chance.