Any brush variant that has blending and smearing characteristics will paint white on transparent areas of a Layer.
The Artists' Oils Blender Bristle is one that, in its default state, blends existing color. Since there's no existing color on transparent areas of a Layer, that's why you're seeing white in the brush stroke.
Try this and see if it helps:
1. On the Property Bar, move the Blend slider to 0%. This will eliminate the blending and the brush stroke may appear darker as it's not mixing/blending with other (lighter) color.Save your file frequently in Painter's native RIFF format and for the first save, use File > Save As. Then for following saves, use File > Iterative Save and Painter will automatically add a number at the end of the file name, a new number each time you use File > Iterative Save. Now you'll have a series of numbered versions and if anything goes wrong, you can go back to the last saved version and continue.
2. To control how dark the brush stroke is, adjust the Opacity slider lower until you get the result you want.
The numbered series of files will look like this:
my_painting.rif (first save using File > Save As)
my_painting_001.rif (second save using File > Iterative Save)
my_painting_002.rif (third save using File > Iterative Save)
.... and so on.
Also read this tutorial at PixelAlley: