Does summoning sickness prevent a creature from blocking?

Summoning sickness — formally called the "haste" rule — restricts what a creature can do when it first comes under your control. Specifically, CR 302.6 states that a creature cannot attack and cannot use activated abilities that include the tap symbol (T) or untap symbol (Q) in their cost unless it has been under your control since the beginning of your most recent turn.

Blocking is governed entirely by a different part of the rules. During the declare blockers step (CR 509.1), a defending player may assign any untapped creature they control as a blocker. There is no requirement that the blocking creature has been under your control for any length of time — only that it is untapped and has no other restriction (such as a "can't block" ability) preventing it.

So a creature that entered the battlefield this turn — even one played as a flash creature mid-combat — can legally block in that same combat, as long as it is untapped and has no relevant restrictions on blocking.

Example: Your opponent attacks with a 3/3 on their turn. You cast a 2/4 Walls creature at instant speed during the declare attackers step. Even though the 2/4 just entered the battlefield this turn (and therefore has summoning sickness), you may immediately declare it as a blocker in the declare blockers step.

The only way summoning sickness could indirectly matter for blocking is if a creature had an activated ability using T to give itself or other creatures a relevant combat ability — but the blocking action itself is never restricted by summoning sickness.