when you cap monitors it's pretty typical with an older model like that to have to redo all the white balancing. I never worked with a 4900 before, considering its age I'm going to say there's no Contrast adjustment on it right, you just have to adjust all your cutoffs and drives manually?
start by turning the machine off (see: monitor off) and set all your adjustment pots on the neckboard all the way down, then get a feel for how much they turn by turning them all the way up. then go all the way back down, and turn them about 1/4 of the way. turn the Screen adjustment all the way down, and set the Focus at the middle. turn the game back on. step up the Screen adjustment about 1/3 of the way, what does the screen look like now?
if it's still a scrambled mess of blue, I would start inspecting the neckboard next. I personally desolder all transistors and adjustment pots, scrape whatever flux is there off with a dental pick, then reinstall everything using fresh solder. be mindful of what adjustment goes where, because cutoffs and drives will use different resistances.
assuming you do everything I said, you will effectively wipe all your pots and eliminate any potentially halfassed solder work on the neckboard.
if you still have an all blue screen, which I doubt you have 2 tubes that magically formed the same issue just by capping it, then you will want to find someone with a rejuvenator to at least test it. if you have a very dark screen, then you need to start turning up your cutoffs and drives accordingly. have a mirror handy for this, cause it's going to take a lot of tinkering to get a pure white picture. you want to get these as bright and vibrant as you can before it starts smearing, without any bias of red, green or blue present. your Brightness and Screen adjustments should be set just high enough that you don't see any pixels in a black background at all.
that's about all I have to share about this, hopefully you get it working.