Does protection from white prevent a white creature from being blocked by white creatures?

Protection from a quality (here, white) grants four specific shields, remembered by the acronym DEBT: the creature can't be Damaged by white sources, can't be Enchanted/Equipped by white things, can't be Blocked by white creatures, and can't be Targeted by white spells or abilities (CR 702.16b).

So if your creature has protection from white, it cannot be blocked by any white creature your opponent controls. White blockers simply cannot be legally assigned to block it during the declare blockers step (CR 509.1b). This is a restriction on who can block the protected creature.

Protection does not work in reverse: it does not prevent a white creature from being blocked by other creatures. A white creature without protection from white can be blocked normally by any creature with a legal blocking ability.

Example: You attack with a Paladin en-Vec (which has protection from black and red) — this has no bearing on white. But if your creature instead has protection from white, your opponent's White Knight (a white creature) cannot be declared as a blocker against it. Your opponent's Black Knight (a black creature) can still block it normally.

In summary: protection from white on a creature means that creature is unblockable by white creatures. It says nothing about whether white creatures themselves can be blocked.