If your opponent casts a creature spell, then when the spell resolves the creature will enter the battlefield (C.R. 608.3), then players get priority (C.R. 117.3b). (Once you get priority, you can cast Cyclonic Rift at that time [C.R. 117.1a].) All this happens during the declare attackers step (so well before blockers are declared), and all this doesn't end the declare attackers step (which generally happens only if all players pass in a row on an empty stack) (C.R. 506.1, 508.2a; see also C.R. 500.2).
If your opponent casts a spell (including a creature with flash), and it resolves, then players get priority. (For instance, you can now cast Cyclonic Rift at this time.) All this happens during the declare attackers step, so well before blockers are declared.
Thank you. So the moment a blocker is assigned, the attacker is considered blocked, even if I cast CR at this point?
When blockers are declared in the turn based action of the declare blockers step, one of the last things happening is, that attackers that have been assigned a blocker, become blocked, and attackers that don't have any blockers assigned to them become unblocked. Removing the blokcer after that point will not change the "blocked" designation of the attacker. The attacker remains blocked until it is removed from combat or the combat phase ends, which ever comes first.