.

but all that comes out is a more or less dark gray core combined with a white blade.

This is probably not what you want, but it is the best you can get with this mod. Each laser blade is colored in order from inside to outside, so a black core in a white blade will look like a grayish core.

I've created a new laser blade model type 2 that solves this problem. This model is available in 1.19-5.3.2.0.
You can get this model with an easy upgrade recipe.