Land in the Adirondack and Catskill regions of NY are to ‘be forever kept as wild forest land,’ and the state is disallowed by law to sell it or develop on it.
The Blue Line in New York refers to the perimeters of the Catskill and Adirondack parks.
The state constitution requires that any property owned or acquired by the state in those parks, as they don’t own all the land within the Blue Line, "be forever kept as wild forest lands" and prohibits it from selling or transferring them in any way.
The constitution may be amended to allow specific transactions, though.
This rule got its beginning when the Forest Preserve was first introduced to the US. The initial Blue Line region designated particular counties in the state as places where Forest Preserve could be acquired in the future.
The main portion of the area within the line was drawn up in 1904. They were previously drawn by counties, but that didn’t work as well as drawing boundaries through a combination of pre-existing political boundaries and old survey lot lines, streams and railroad rights-of-way.
A simple majority can add land to either of the lines, but taking land away requires amending the constitution which needs two successive legislative sessions to approve of it.
In case you were wondering why it’s called the blue line, that one’s pretty easy: blue ink was used when they were first drawn on state maps, starting a tradition that persists to this day.