Blend is defunct.

Yes, it's the old thing of it painting in white instead of blending what's below. 
Yes I have pick-up underlying colour selected. 
No There's nothing special about the layers below, or the layer I'm trying to use..
No watercolour or oils etc.  They are all default and ignore.
I've not messed around with the blend options button on the toolbar.

Previously the Just add water brush would work fine. but now in Painter 2020 I went to try and blend as I usually do and I got white instead of transparent.

I tried everything except for restoring to factory defaults, because I have lots of brush packs installed and don't want to go through the whole thing of trying to re-install them.  Well that or uninstalling painter and forgetting it ever existed and moving on with my life.

Can anyone help please?