the blaze fire is still red and will not turn blue

This is a feature request. Blazes don't turn into "soul fire blazes".

But blazes are a fire, and technically they should turn blue?

Blue Blazes would be really interesting, but I agree it's a feature request. Think of it this way:

If I spawn a Mooshroom using  a Spawn Egg in a Plains Biome, it's still going to be a Mooshroom. It doesn't become a regular cow just because I used the Spawn Egg on the grass.