Can a creature that just entered the battlefield block in the same turn if it has haste?

Haste (CR 702.10) removes two specific restrictions: it lets a creature attack and lets it activate loyalty or tap abilities the same turn it enters the battlefield. Blocking is a completely separate action governed by different rules, and haste does not interact with blocking at all.

The ability to block is not restricted by how recently a creature entered the battlefield. Any creature without a specific restriction (such as being tapped or having a rule that prevents it from blocking) may block during the declare blockers step (CR 509.1), even if it just entered the battlefield that same turn — with or without haste.

So a creature without haste that enters the battlefield can still block immediately. Haste is simply irrelevant to blocking. CR 702.10a states: 'Haste lets a creature attack and lets a player activate its abilities with the tap symbol or untap symbol even if that creature hasn't been controlled by you since the beginning of your turn.'

Concrete example: Your opponent attacks you with a 3/3. You cast a vanilla 2/2 with haste during your opponent's combat phase (via flash or some instant-speed method). That 2/2 can block the 3/3 normally — but this is because any creature can block, not because of haste. If the 2/2 lacked haste, it could still block; haste made no difference here.

In summary: haste never grants or restricts blocking. Blocking is always available to untapped creatures with no blocking restrictions, regardless of when they entered the battlefield (CR 509.1a).