Learn English – I can’t say I blame you vs I don’t blame you

Both are used idiomatically to mean

Your actions/feelings are understandable or correct.

See, for example, the definition at Oxford Dictionaries.

This generally has the implication that the other person did do something, but that action was not actually blameworthy:

I can't say I blame you or I don't blame you for breaking that vase--it was the ugliest, cheapest thing I've ever seen, and I would have 'accidentally' dropped it, too.

If, however, something truly bad happened, and you want to make clear that you don't hold the person you're talking to responsible, then you would only use the direct I don't blame you:

Of course I don't blame you for the broken vase--the table gave way under it because it was defective, and you had no way of knowing that.

In the latter case, using I can't say I blame you would ring of the idiomatic use, and so would suggest that you don't actually believe the story about the defective table; kind of a verbal wink-wink, nudge-nudge.