How accurate is credit karma?

No clue. I do know it matches the score that a credit card also gives me.

My understanding is that there are different "blends" of FICO depending on the use? For example, the one that Discover and Barclay's gives is a CC FICO, which is always within 3 points of my CK score.

I've also heard that (Coach G may be able to confirm) that there is a blend for auto loans. I have not applied for an auto loan since I began using CK, but a friend of mine did and so did his wife, and his and her CK scores were about 15 points lower than the score the dealer pulled.

I did recently apply for a mortgage, and the mortgage score was about 40 points lower than my CK score (and lower than my Discover FICO). The mortgage officer told me that a mortgage FICO looks at different things.