Can you flash in a creature during the declare attackers step before blockers are declared?

After attackers are declared and the active player has passed priority, both players receive priority in succession during the Declare Attackers step. Any player holding priority may cast a spell or activate an ability at instant speed — including casting a creature with Flash (CR 116.1, CR 509.4).

However, the creature you flash in cannot block in the current combat. Blockers are only declared during the Declare Blockers step (CR 509.1, CR 116.1b), and a creature must be on the battlefield and untapped at the moment blockers are declared to be eligible. Even if you flash your creature in during the Declare Attackers step, the game will not return to a Declare Blockers step after it is already in progress — it moves forward (CR 509.4).

The key point: flashing in during the Declare Attackers step means the Declare Blockers step hasn't happened yet, so your creature can legally block — provided it arrives before the Declare Blockers step begins. If you flash it in while still in the Declare Attackers step, it will be eligible to block when the Declare Blockers step starts, as long as it has no summoning sickness restriction on blocking (blocking does not require the creature to have been under your control since your upkeep — CR 302.6).

Example: Your opponent attacks with a 3/3. After they declare their attacker and pass priority, you flash in a 2/2 with Flash. When the Declare Blockers step begins, your 2/2 is on the battlefield and untapped, so you may legally declare it as a blocker — even though it just entered this turn.

Summoning sickness (CR 302.6) only prevents a creature from attacking or using tap-symbol abilities; it does not prevent blocking. So a creature flashed in during the Declare Attackers step is perfectly eligible to block in the immediately following Declare Blockers step.