Altium shows little black blocks on my GND pads and tracks
Any chance you're using AD17? They introduced a feature which allows you to set a color for your nets in the schematic. This color is then transferred to the PCB and this is what you're seeing.
You might want to remove the color from the net in the schematic, transfer it to the PCB and it should be gone.

Alternatively, you can also set a net color in the PCB.