When you say the word "blunder" are you referring to a blunder as defined by an engine or a blunder as defined by a human?
Game analysis
When you say "blunder" are you referring to a blunder as defined by an engine or a blunder as defined by a human?

After the trading of the Queens, black has undoubtedly come out better from the opening. You have the two Bishops, better development, and a more central and active king, all pluses for the coming endgame.
However, white misses some a key chance...b5 I believe is a minor positional mistake due to the weakening of the c5 square. After white got in a3 and b4, his plan should be to place the knight on this beautiful outpost with nb3-c5. This is why the computer wanted you to play rc3 rather than rc2, to keep the knight from playing to b3.
Other than, a very good game from black! White really lost his chance to play for an advantage by quickly trading off into this endgame. Its symmetrical pawn structure, and his inability to ever develop and contest the c-file ultimately gives most all the chances to black.

When you say "blunder" are you referring to a blunder as defined by an engine or a blunder as defined by a human?

When you say "blunder" are you referring to a blunder as defined by an engine or a blunder as defined by a human?

The Caro requires that White has played e4. This is still a Slav. Your opponent opened very passively and you took advantage of that. Good job!
Your notes confuse me a bit because I want to match the numbers with move numbers and they do not match. I guess they are note numbers. Maybe it would be better to give them letters so that they are not confused with move numbers.
If I may be a bit critical, there are a couple of moves in there that I think could stand improvement. First, what does 13...b5 achieve for you? You already mentioned having the open c-file. Why not go ahead and get your rooks on it as quickly as possible?
The reason the engine prefers 15...Rc3 over your choice of ...Rc2 is that your move allows White to play Nb3 -- which White overlooks, anyway. White could have gummed up your c-file with Nb3/Nc5. This is another reason that your ...b5 was a positional error -- it left a hole at c5.
Another move I wonder about is the move 19...Ke8 -- this is a questionable move, IMHO. I assume you were worried about your kingside pawns, but the truth is they are not attacked and can easily defend themselves.
Overall you took advantage of your opponent's passivity and came out on top.

The Caro requires that White has played e4. This is still a Slav. Your opponent opened very passively and you took advantage of that. Good job!
Your notes confuse me a bit because I want to match the numbers with move numbers and they do not match. I guess they are note numbers. Maybe it would be better to give them letters so that they are not confused with move numbers.
If I may be a bit critical, there are a couple of moves in there that I think could stand improvement. First, what does 13...b5 achieve for you? You already mentioned having the open c-file. Why not go ahead and get your rooks on it as quickly as possible?
The reason the engine prefers 15...Rc3 over your choice of ...Rc2 is that your move allows White to play Nb3 -- which White overlooks, anyway. White could have gummed up your c-file with Nb3/Nc5. This is another reason that your ...b5 was a positional error -- it left a hole at c5.
Another move I wonder about is the move 19...Ke8 -- this is a questionable move, IMHO. I assume you were worried about your kingside pawns, but the truth is they are not attacked and can easily defend themselves.
Overall you took advantage of your opponent's passivity and came out on top.
Great feedback, thanks.
My point is that the move gives me a similar structure to the caro kann, not that I'm now in the the opening. Sorry if I was unclear.
I was under the impression the numbers in my comments matched the game moves. I musta screwed that up.
B5 was grabbing space and creating a strong pawn structure. I wasn't worried about c5 because the Bishop could move it off, especially once the rooks hit the column. It seemed to me that a clash there would result in my obtaining a passed pawn. I agree it was a mistake, the pawns could have been moved later. I'll take a look at your thoughts about C5, but b5 was at least a time waster.
Regarding Re3 instead of re2; good insight. thanks.
19 is actually ke7, and my comments acknowledge it as my worse move of the game. I was actually thinking to get my king to safety. A bad move all around
AND...I'd thought I'd cleverly grabbed the initiative, but you're right, my partner was simply too passive.
Thanks again, Mouselip. Very helpful feedback.
Don't usually do this. In my games, the other player is usually about my level, which means the 11th or 12th blunder decides the game, and that's not interesting.
2 things, tho. they're about 100 points above me, and no blunders. (Mistakes don't count as blunders so phffffffft!)
Here are my comments. (I'm playing black)
They resign a few moves later when I start knocking off pawns.