Im not sure how hot these bulbs burn at put it is possible to color them with a trasnparent blue marker if they dont burn hot. And if they do, Iv seen these coveres that you can put over the bulbs, but these are for the H4 headlight bulb. I imagine you can custom fab something up that would cover the bulb and mae it blue.
Or better yet, take that marker and color the back of the guage cluster where light shines through, making it blue. Id try it with a small part of the speedo in case it doesnt work out and that way the whole thing isnt ruined.