The short answer is yes any blue can be removed it will just be a little time consuming. If painted on or its a piece of vinyl you can just use a razor blade and slowly scrape it off. Both should come off with the razor blade just take your time so as to not scratch up the back of the tank.
If you really want that tank you can do it, all it takes is a little elbow grease and a razorblade!
disclaimer: I've done it before, it SUCKS. a good paint job will take hours to scrape off