"Black letter law" refers to any law which is strongly established, recognized by everyone in the legal sector in that law's jurisdiction, and rarely, if ever disputed. Black letter laws are laws which lie in the base of any law definitions, and serve as a foundation for other laws to be established on - for example, the definition of a contract is considered a black letter law, as nobody disputes what a contract is, the details surrounding engaging in a contract, or the punishment for violating a contract's claims.
A common misconception is that the name "black letter law" came from an association with Black Law's Dictionary, an issue published about a century ago - in reality, the first occurrence of the term can be seen in the 1847 case of Naglee v. Ingersoll, held in the Pennsylvania Supreme Court - a case which took place about 50 years before the first publication of Black Law's Dictionary.
It's not uncommon for black letter laws to not be specifically written down, instead being generally recognized and well-known - this includes some of the most basic definitions used in the legal sector. Some attorneys have notoriously attempted to exploit the definition of a black letter law, using it to claim that some rules which could benefit their party's case were considered black letter laws and thus should not be disputed, when it was not the case. It's also not rare for attorneys to wrongly interpret black letter laws due to changes in their jurisdiction of operation.