Haste is irrelevant to blocking. A creature can block on the very turn it enters the battlefield — with or without haste — because the summoning sickness rule (CR 302.6) only restricts a creature from attacking and from activating abilities with the tap or untap symbol. It places no restriction whatsoever on blocking.
Haste (CR 702.10) simply overrides the summoning sickness restriction for attacking and tap/untap abilities. Since blocking was never restricted by summoning sickness in the first place, haste has no meaningful interaction with a creature's ability to block.
The declare blockers step (CR 509.1) requires only that a blocking creature be untapped and able to block (not affected by a rule or effect preventing it from blocking). There is no 'has been under your control since the beginning of your turn' requirement for blockers.
Example: Your opponent attacks you with a 3/3. You cast Llanowar Elves on your first main phase (no haste), giving it haste via an Anthem effect doesn't matter — even without haste, Llanowar Elves can still block that 3/3 in the declare blockers step, because summoning sickness never prevented blocking.
Bottom line: giving a creature haste does not grant or restrict blocking in any way. Any untapped, eligible creature may block on the turn it enters the battlefield, haste or not.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.