is there another version of jit.alphablend that preserves transparency?

The way jit.alphablend seems to work is that any cells that were fully transparent on input are converted to fully opaque black on output, i.e 255 0 0 0.

I would prefer if the full transparency of the cell was preserved, e.g. 0 0 0 0.

This would mean I could take the output and use it again for alphablending further down the line.

I'm having a go at writing my own abstraction to do this, but any advice, or links to an object that may do this already, would be appreciated.