Just speculation, but: A blue bulb, depending on its exact characteristics, might produce a higher-contrast print on variable contrast (VC) paper than would a white bulb. It's conceivable that the enlarger's original owner liked this and so used a blue bulb to get this effect. Of course, I have no way of knowing this is so; you'd have to run some tests to even verify that the bulb has this effect. If you know who the original owner was, you could ask.
Blue or cyan filters have no effect on VC or non-VC paper. They block light outside of the paper's sensitivity spectrum. The paper doesn't know or care