Black Letter Law refers to the basic standard elements or principles of law, which are generally known and free from doubt or dispute. It describes the basic principles of law that are accepted by a majority of judges in most states. For example, it can be the standard elements for a contract or the technical definition of assault. In American legal system it also means mean well-established case law.
Example of a Case law using the term.
It is black letter law that it is a party's first obligation to seek any relief that might fairly have been thought available in the district court before seeking it on appeal. [Beaulieu v. United States, IRS, 865 F.2d 1351, 1352 (1st Cir. Mass. 1989)]