as outlined in your other thread, this could mean any number of things.
one is a bias/drive pot adjustment that isn't correct. this could either mean the red and green are turned down, or the blue is turned up.
the other, which is probably the more common cause, is the tube has a short in the blue gun or the red and green guns are weak. a monitor rejuvenator can test for this condition and potentially repair it for you.
another option is you have failing color drive transistors on the neckboard... it was suggested in another thread that the color in question could've been a shorted or bad drive transistor. logical way to diagnose this, if none of the above worked, is to swap the bad primary color in question (in this case, blue) with the red let's say. does the screen now carry a red tint?
the last is cap kit. everyone here will tell you that before they even help you in a lot of cases, as if you have that kit up your ass or you have the soldering skills required to even install it. lol
my rule is that if the tube is larger than 25" and it has problems, pass on it. 19s and 25s are very easy to come by, at least where I am.