the quality of the tubes on these were kind of bad, but I don't think your missing blue is tube-related. the neckboards on these (and the K7400 and K7500) all had a flaw where the color drive transistors would get hot enough to lift the traces on the back of the neckboard. in turn, the solder pads can often times break off the trace too. there goes your colors.

I suppose in other really rare cases the color adjustment pots can fail too, particularly the white knob variety (wtf is it with the white knobs?) but I doubt that.
if your transistor has enough leg left sticking out the back, what I've done with these and also with WG D9200s (which also have the same heat problem) is you bend the legs at a 90 degree angle, often times try to face it the same direction as the trace, what this will do is eliminate the flexing that can pull the traces up more. then after that you can either bridge traces with wire or you can scrape the outer layer of the trace and flow some solder on it to reconnect to the transistor legs or solder pad if those are still intact.
I should really do a pictorial guide on this sometime, cause it's saved my ass a few times with a few of our games at work lol
the color transistors, facing the back of the neckboard, go in order of Red Green and then Blue (the 3rd one by the edge). get a meter out and check continuity from the 3 transistor legs to the next points in circuit to verify that the traces are good or bad or whatever.