Its ability will destroy all creatures it’s currently blocking or blocked by, not necessarily the creatures it had blocked or become blocked by during that combat phase. For example, if Kjeldoran Frostbeast is dealt lethal combat damage and regenerates, it’s removed from combat. Its ability will trigger at end of combat, but since it’s no longer blocking or blocked by any creatures, the ability won’t do anything.

Kjeldoran Frostbeast’s ability triggers only if it’s still on the battlefield when the end of combat step begins (after the combat damage step). For example, if it blocks a 7/7 creature and is destroyed, its ability won’t trigger at all. The 7/7 will remain on the battlefield.