I Have A Question About Paper Bills

By "solid block", do you mean a serial number with all numbers the same? If so, then yes, they are issued without favor or theft, but they are understandably uncommon to find.

"Solids", i.e. a serial number where all the digits are the same, are rather rare. It's pure statistics.

There are eight digits in a serial number on a modern US note. These digits can, essentially be treated as random numbers. With only ten possible solid notes in a series from 00000000 to 99999999, this in turn means that there is a one in 1000000 chance that a banknote selected at random will have a solid serial number - a far lower probability than other "cool patterns" like semi-solids, repeaters, or radars.

BEP employees essentially cannot just "nab" notes. It's not like they give away free samples to their workers or something like that. This is, after all, the actual place where all the government's physical money is printed. The level of scrutiny applied to BEP workers who handle banknote printing would boggle your mind. What is happening, however, is a far higher probability of some random member of the population noticing a solid note and saying "wow that's cool I'll keep it", so they're much more likely to get filtered out before you see them.

The odds of finding a solid serial number are lower - ~ 1 in 10,000,000. They are not likely to be encountered.