To the extent that one believes that Bush was wrong for ordering the invasion of Iraq, blaming him for all time for the failures of his Administration is appropriate.
Blaming him for anything and everything that has happened or will happen in regards to Iraq may, as you suggest, be the way a lot of people's brains work, but it is not justifiable.
If, when Obama leaves office the ACA is operating relatively smoothly, but his successor botches up a key component of it which renders it a failure, it would not be appropriate to blame Obama for its ultimate failure. It would be inviting to do so since he's the one who launched it in the first place, but unless the botched component could not possibly have been executed properly and therefore the entire program was doomed from the start, an assertion that Obama was to blame would be a fallacy.
But there is such a thing as intervening causation and in fact the implications of such causes in determining liability (blame) are addressed in a legal doctrine. The current crisis reasonably invites discussion of the entire history of our involvement in Iraq, and so addressing the failures of Bush & Co as regards this involvement is not irrelevant. However, focusing on whatever blame Bush may deserve, when the issue at hand is how Obama may have helped create the current situation, or more immediately, how he is reacting to it, is pointless, unless, of course, the point is to divert focus away from Obama.