I personally dont think that it was a virus. I mean i logged in several times into my bankaccount too and there was everything fine. But maybe on the other side its easier to steal your btcs than your "real money". Maybe i just got hacked and thats it. Either way for me the wallet at blockchaininfo is dead.
No, blockchain isnt dead since its being used for how many thousands of bitcoin users since this wallet is easy to use when it comes to online transactions you are just unlucky to get hacked by someone. They do have your logged in information which supposedly you do get from virus or keylogger. Ive been using this for years now and i dont have experience such problem on losing funds.For now reformat your pc install antivirus then dont tend to download random things on internet.You can say that you dont have virus but actually its just sitting down there on your pc without any knowledge or awareness.
Im not sure it's a keylogger as you have to authenticate whenever you log in through email (check the security log of your email op to see if there's anything there of other IP addresses that have been used)?
2FA is always a good idea to enable along with IP restrictions.
If you have a good enough antivirus, you shouldn't need to go to the hassle of reinstalling the OS. Just run a full system scan from it and any .exe files it finds to be dangerous or not very well known, it'll destroy/quarantine them for you.