Welcome to PhotonLexicon!
Very cool; I remember seeing these fixtures as a kid in some of my friends' basements back in the day.
Are you concerned with keeping things as close to original / vintage as possible, or are you just looking for a nice, bright bulb with a pleasing color?
If you don't care about authenticity, I'd suggest replacing all the bulbs inside your units with LEDs. You can find a wide assortment of colors in the C7 socket design. While you may not be able to match the original incandescent bulbs exactly, I'm sure you can find something that is very close. If you replace all the bulbs (not just the blue ones), I suspect you'll never notice any slight color difference from the original, since all of the new bulbs will match exactly. And with LEDs, it's highly unlikely that you'll ever need to change a bulb again.
Some examples:
https://www.amazon.com/LightSupply-M...08BC4ZC9Y?th=1
https://www.amazon.com/ILAMIQI-Cande.../dp/B08ZK8HNTR
https://www.amazon.com/Christmas-Mul.../dp/B09DT2F12L
https://www.amazon.com/Brightown-Rep.../dp/B0757N88LW
The first two options are specifically listed as "dimmable", which might be an important feature if the organ is designed to have an analog response to sound.
Alternately, if you insist on sticking with the original incandescent bulb design, then you might need to post a picture of one. There are lots of different coatings for colored bulbs, plus the dye tends to fade over time, so matching it exactly will be a challenge.
Here too, your best option might be to replace ALL of the blue bulbs, rather than just the bad one. This should minimize any differences between the vintage bulbs from 45+ years ago and the modern replacements.
Adam
PS: Did you happen to notice
this thread about old-school light boxes? Very similar to your color organ.