It's dangerous and definitely a scam.
A common way to do this is as follows. Using your username and password, the guy will secretly retrieve the recovery phrase from the wallet. It will then let you change the password. Then, he will invent a story to convince you to acquire Bitcoins and put them in the wallet (or he may just wait until you decide to do it on your own). You will think it's safe because he does not know the new password. However, using the recovery phrase, he will be able to take all the pieces of the wallet, even without the password.
If he really wanted to have access to a Blockchain wallet, nothing would prevent him from making one himself.