it's "normal"
Some explanations :
Because Black Ink use a lot of cache when you draw especially when you didn't transform your canvas ( with a pan/zoom or an Undo/Redo ) and also because it's not a
Vector but a
Rasterizer graphic program
All you see is pixel perfect. But when a full update of the view is required Black Ink use
mip version of the image and then a little blur can occur due the nonalignment of pixel between the canvas and the View.
It's a little technical and I hope you understand..
Anyway don't worry because this little "blur" effect isn't present when your save or export your final picture.