The method User::isBlocked() attempts to answer two questions:
- Does the user have a block?
- Is the user prevented from performing this action?
The method can answer #1, but it cannot answer #2. Since User::getBlock() can also answer #1, this method is redundant. The method cannot answer #2 because there is not enough context in order to answer that question.
If access is being checked against a Title object, all access checks can be performed with PermissionManager::userCan() which will also check the user's blocks.
If performing all access checks is not desirable, using PermissionManager::isBlockedFrom() is also acceptable for only checking if the user is blocked. This method does not determine if the action is allowed, only that the user's block applies to that Title.
If access is being checked without an existing Title, User::getBlock() can be used to get the user's block. Then Block::appliesToRight() can be used to determine if the block applies explicitly to a right (or returns null if it is unknown or false if explicitly allowed). If the user is creating a new Title, but the text of the title is not yet known (as in the case of Wikibase), access should be checked with Block::appliesToNamespace().