In an era when the postal regulations repeatedly insisted that only black printer's ink should be used to obliterate stamps..
The above quote is from the U.S. Philatelic Classics Society; Volume 50 Chronicle 180 "Color Cancellations on U.S. Official Stamps 1873-1874
" by Alan C Campbell
I couldn't find the actual regulation against colored inks, just that reference to it, which may be why black is more common than other colors of ink. Ultramarine was the most expencive color of ink at the time these stamps were printed from what I've read on this board.