When can I cast Glyph of Destruction?
Yes, your wall has to be declared a blocker before you can target it with that glyph. Your wall remains a blocker (and thus a legal target) until the end of that combat phase, though you'll have to target it before the Combat Damage step if you want it to actually deal that +10 damage in combat.

A creature is only considered blocking or attacking once it is declared as such, and ceases to be those things once combat is over. There is no such thing as a blocking or attacking creature outside of combat.