Well, Curt gave me some good advice. Disconnect the blue video feed from the RGB card.
I did this via the internal connection – the wire between the blue CRT amp card and the RGB card. The blue indeed did go dark. I then swapped the blue to the green signal, which I knew was good. The blue was nice and solid. Then a swap back to the real blue — and again, solid. I suspect that one of the connectors was dirty and was causing trouble.
OR there is a possibility that my RGB card is intermittent or something. However, I’ll cross that bridge when I get to it.
Right now it appears all systems go.