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.

Sometimes the two are interchangeable, sometimes not. It depends on context.

Generally speaking "I can't say I blame you" is less direct than "I don't blame you". Saying "I can't say I blame you" might acknowledge a behaviour without taking any stand on whether it is right, wrong, or acceptable. In comparison saying "I don't blame you" suggests some agreement or acceptance.

Any two different phrases are, well different. They always have some difference in the set of their possible connotations.

Whether two phrases are interchangeable depends on (1) the context and (2) just what you mean by "interchangeable", that is, (1) how different their meanings are in that given context, and (2) whether you consider that amount of difference sufficiently significant in that context to decide that they are not interchangeable.

IOW, without (1) a clear understanding of the context of use and (2) a clear definition of what constitutes interchangeableness in that context, the question cannot be answered usefully - it is then just opinion-based.

In general, "I can't say that..." means that you either (1) don't really know for sure that... or, less often, (2) you know it but you don't feel that you can say it.

There seems to be a subtle distinction between the phrases which means they are not always interchangeable.

"I don't blame you" can have two meanings. Either "you did it but I do not fault you for it," or "I don't think you did it". In other words, it can mean that you were responsible but justified or that you were not responsible.

I don't blame you for the world financial crisis. You just wanted to buy a house.

I don't blame you for punching Derek. He insulted your mother.

"I can't say that I blame you" seems to be used only to mean the you were responsible but justified, not that you were not responsible. It is often used as a response to a confession.