If you have an old school TV repair center in your town, give them a call to see if they have a rejuvenator. Most of the time they'll check/test/rejuve it for you for next to nothing... even if they do charge a bit for it (I wouldn't pay more than $20), it's still cheaper than getting another monitor/tube.
FWIW, I had a... I think it was 4600 in my Pole Position and the tube had a bad red gun. Hooked it up to the rejuve, ran a few "clear shorts" and "clean and balance" and she was good as new.
If you want to do a quick and dirty test of the tube, you can short the gun in question (blue in your case) very briefly. That's what I did to mine before I borrowed the rejuvenator from work. Shorted green, screen turned green. Shorted blue, screen turned blue. Shorted red... nothing. YMMV and do it at your own risk.