Haste is not needed to block with a newly played creature, and conversely, haste does not grant any special blocking ability. The rule about 'summoning sickness' (CR 302.6) only restricts a creature from attacking or using activated abilities with the tap/untap symbol if it hasn't been under your control since the start of your most recent turn. Blocking has no such restriction.
CR 509.1 explains that during the declare blockers step, the defending player may block with any untapped creature they control — there is no requirement that the creature has been under their control for any particular length of time. So a creature played this very turn can block freely, with or without haste.
Haste (CR 702.10) specifically removes the summoning sickness limitation for attacking and tapping/untapping via activated abilities. It has no interaction with blocking whatsoever, because blocking was never restricted by summoning sickness in the first place.
Concrete example: Your opponent attacks with a 3/3. You cast a 2/4 creature with no abilities this turn. Even though it was just played and has no haste, you can immediately declare it as a blocker to stop that 3/3 — summoning sickness doesn't apply to blocking.
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.