This is likely an error in the part definition. The first thing I would try is export the sketch to a gerber and check the gerber output to see if the black bars are suppressed. If not, you or one of us would need to fix up the pcb svg file.
You would need to make a pull request against the core part in the fritzing-parts repository on github. This should be an easy enough change that you shouldn’t have issues about obsoleting the original part (which isn’t documented as far as I know.)