The Blob Brush tool will not aways 'blob' with a shape (merge with it) even when all the properties are matching and correct (same color and no fill). I will make a shape with a solid fill and no stroke. I will select the Blog Brush tool to paint over it with the same color and it won't merge with the shape as it should. Sometimes it works, sometimes it doesn't. I have tried turning off GPU to see if that helps, but it doesn't. I have never had this problem before.
The only thing that I have noticed is that when a shape is first created with no stroke and a solid fill and I immedately use the Blog Brush it seems to work everytime. However, if I have existing shape with a fill and and stroke and turn the stroke off in order to use it with the blob brush tool it is more likely that it won't work (even though the fill color matches with the Blob Brush color). It's as though the Blob Brush tool is still seeing the original shape attributes even though the Properties panel and elsewhere clearly show that the stroke is off. This is not even consistent though.
System spec and screen grabs are below.
Also, I deleted Illustrator Preferences and the behaviour continues.