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.