Well, so much for my Low Pass idea!!!
a picture's worth a thousand words.
ignore that post, that's not what the cap does , and i apologize.
The black stripe -
That type capacitor is insensitive to which lead is positive or negative.
A capacitor is made from two strips of foil, separated by an insulating film and rolled into a tube.
The two lead wires are each connected to one of the foils.
The black stripe indicates which end connects to the foil that's on the outside of last wrap.
It's not important in your application which way it goes.. Were it a filter, the outside foil ought to be the one grounded.