Does an "as-is" clause in my purchase contract block this kind of claim?

No, and this is a critical point that sellers and their agents frequently misrepresent to discourage legitimate claims. An "as-is" clause in a purchase contract functions to shift the risk of defects that a buyer reasonably could have discovered through their own diligence onto the buyer, but under established California law, it does not excuse a seller from their independent legal duty to disclose known material facts they were actually aware of, and critically, it is not a valid legal defense to active concealment, meaning deliberately hiding or covering up a known problem.

Despite this being settled law, sellers and their real estate agents raise the "as-is" defense constantly in these disputes anyway, likely because many buyers assume it's a complete bar to any claim and simply give up rather than pursuing what may be a genuinely valid case. Don't assume an as-is clause ends your potential claim before actually having the specific facts of your situation evaluated.