Can a creature block if it gained haste the same turn it entered the battlefield?

Haste (CR 702.10) only removes two specific restrictions: it lets a creature attack and lets it activate tap or loyalty abilities the turn it enters the battlefield, without needing to have been under your control since the beginning of your most recent turn. Blocking is governed by entirely separate rules.

Under CR 509.1a, any untapped creature you control may block during the declare blockers step, as long as no other ability specifically says it can't block. There is no rule that prevents a creature from blocking in the same turn it entered the battlefield — with or without haste.

In other words, haste is irrelevant to blocking. A creature that entered the battlefield this turn (even without haste) can already block freely. Haste doesn't grant extra blocking ability; it only lifts the attack and tap-ability restriction imposed by CR 302.6 (summoning sickness).

Concrete example: Your opponent swings with a 3/3. You flash in a Spark Elemental (which has haste) during the declare attackers step or earlier. During declare blockers, you can assign your new Spark Elemental to block that 3/3. The haste is irrelevant to this choice — even a freshly entered creature without haste could block.

So the answer is simply: yes, a creature with haste can block the same turn it enters, but so can any creature without haste. Haste neither grants nor restricts blocking ability.