cl-block is an autoloaded Lisp macro in `cl-macs.el'.

(cl-block NAME &rest BODY)

Define a lexically-scoped block named NAME.
NAME may be any symbol. Code inside the BODY forms can call `cl-return-from'
to jump prematurely out of the block. This differs from `catch' and `throw'
in two respects: First, the NAME is an unevaluated symbol rather than a
quoted symbol or other form; and second, NAME is lexically rather than
dynamically scoped: Only references to it within BODY will work. These
references may appear inside macro expansions, but not inside functions
called from BODY.