I guess when you said black cartos you were talking about the Boge 510 style. I sometimes fill mine from the top by dripping, as some have said, but the best way is the condom method, especially with a new one. You get full saturation from the condom method.
Cartos come with silicone caps that seal the ends. Use the longer one to fill the carto. Drip into the longer silicone cap until it's about 1/3 to 1/2 full of juice and then put the cap on the open end of the carto. Put the short cap on the battery end and lay it horizontally on a folded paper towel. Leave it for 30 minutes or so and then remove the caps and blow it out lightly through the open end. You probably won't have any juice in the center tube to blow out, but it's a good idea to do that just in case.
This guy does it a little differently, but you can put the cap on either end and accomplish the fill either way.