Historical Rules · 11 November 2016
Rule 506.6b
Official source text from the 11 November 2016 Rules release
A spell that states it may be cast “only before (or after) blockers are declared” is referring to the turn-based action of declaring blockers. It may be cast only before (or after) the declare blockers step begins, regardless of whether any blockers are actually declared. (See rule 509.)