The reason black lights work is there are polymers in most modern paper. These reflect the light causing the glow. You can get paper that does not contain polymers, but it is hard to find.
For me, it iis very hard to see the print -vs- photocopy difference without looking at both sides of the paper. If you can see both sides, you would see a lot more ink bleed through on something printed on a press. This also varies with how much ink is applied to the head of the press. In rare cases, you can actually see the physical indent made by the press if it is a hard strike on the paper.
Another thing to watch for is that most modern printers use a dot matrix method to print. If you look at it under high magnification you can see that the images are comprised of many little dots instead of a continuous line. If you want, I can try and capture this in some photos with a macro lense. Once you’ve seen it, you will understand better.
Unless the counterfeiter is very cleaver, with a little experience, you can usually spot the fakes. Sometimes, just have to compare the real and fake side by side to be certain. For example, a llot of the fake combustible cartridge packets out there were made on polymer free paper and were printed on presses, you jjust have To “know” the difference.
I hope this helps,