A few things can affect your tension mid bobbin as well as between bobbins. A blob of lint getting stuck in the bobbin case spring can cause it. Change in tension on how the bobbin was wound and sadly, there is nothing you can really do about that one. It is very difficult to keep the tension uniform on your own wound bobbins. Heck even the prewounds can vary. Superior superbobs are notorious for having bad tension for the last bit of the bobbin. Some people can quilt with them down to the last centimeter of thread and others simply stop and save the end bits for hand sewing bindings. Some less than great quality threads can vary in thickness on the same spool and affect your tension. I always recheck tension with every bobbin change. Some quilters will absolutely not use different colored threads in bobbin and top because this is such a very common problem and can happen at any time with seemingly no rhyme or reason.
The good news is you can take a micron pigma pen in black to camouflage the gray pokies on the back. The occasional pokie will not compromise the quilt's integrity so you can also live with it. Your wall hanging is very pretty and the occasional gray pokie on the back shouldn't be of any concern. It would be a much bigger deal if you were getting black pokies on the front. I am curious, is there a story behind it as to why the girl is blindfolded?