Are you sure the email was sent from blockchain.com? Are you sure the fund was stolen before opening the email? Maybe, that was a phishing email. (This is just a guess)
How did you login to your blockchain.com account after seeing that email? Did you go to blockchain.com as before or you clicked on a link sent via email?
I fully am conscious of that happening and it was a text that contacted me and I never replied to it anyway, it just prompted me to log on with a different laptop to see what was going on, but the text was genuine regardless and anyway the money had left my wallet as a slept 6 hours earlier.