A flame particle texture (fpt) makes gets inverted and then combined with the original color of the flame to give it's effect.
So when you upload something that's white, it gets inverted to black and black combined with blue = blue. As black doesn't change the original color at all for flames.

The flame you boughtis originally blue, and as the inverted color of the fpt will always get combined with this blue, the only colors you can give this flame are shades of blue. Black is impossible.

If the original flame was black you could've made it any color you wanted with an fpt.

Hope that explains it for you.

I actually forged this flame you bought so my bad, would maybe have been smarter to make it black with a blue fpt, but I wanted to make sure it looked like rain.