I black them out to deter some bozo counterfeiter from using an NGC certification number for one of my coins to make a fake slab. Perhaps I am paranoid, but I don't want that headache of having known fake slabs with a NGC certification number that matches my legitimate coins.
You are not paranoid for opacifying slab numbers and bar codes. I do it myself whenever I remember. It is clear that fakers are already using harvested slab numbers for their fake products. But coin sales sites such as eBay and TPGs like NGC/PCGS could be more readily available resources for the fakers.
More concerning for me is the loss of privacy which could result in questionable to outright nefarious use of such information including tracking. Precious metal collection has privacy as an inherent characteristic; protecting you from shady characters and institutional "shakedown artists"!
Once more, Congratulations and Great Job!