I'm not qualified to answer about the legal rules surrounding confidentiality. (Though, I imagine you have a better handle on California/United States confidentiality than I do anyway.) However, there are a few technical details you should be aware of.
First, transactions in Bitcoin are essentially public. The transaction amounts and the time of the transaction are both public information. That means that if you're forced to disclose the amount and day of the transaction, someone could figure out the transaction that you were paid by.
Second, transactions are linkable. That means that if someone figures out which transaction you were paid by, they could follow the money that you were paid with, and they might be able to identify your client.
Are you sure this is ethical? Wouldn't any opposing party be "obliged to struggle in the dark against unknown forces" with no possible legal redress should your client's actions be deemed malicious or actionable. I thought that attorney client "privilege cannot be used to evade a client's responsibility for the use of legal process". – David Schwartz – 2015-11-25T11:40:53.000