What blockchain wallet you use the mobile app or directly from the blockchain website?

Do you have two-factor authentication or 2FA? Do you whitelist only your IPs?

If you have this it's impossible for someone to login to your account and stole your BTC even you've phished unless if your secret key from 2FA is compromised and you are using the same password as Gmail account(Without 2FA). If they can access your Gmail they can easily retrieve information from your blockchain account by using "Remind me"(Wallet ID) or "Reset 2FA" and use the same password that you use from the phish sites.

Or if you are using the mobile blockchain wallet your phone might be monitored or remotely use by someone to get your 2FA and password from your phone.

Or possibly someone knows your blockchain seed phrase. Where do you save your seed phrase? If you just save it somewhere on your PC or phone (Might be infected) maybe someone stole your saved seed phrase and use it to recover the wallet from blockchain.

Try to ask blockchain support about login IP history to check if there is different IP login into your account.
If there is a different IP then someone has access to your Blockchain account but if none it might be a blockchain bug.