Currently, objc-mode does not take block arguments into account when guessing an 'objc-method-call-cont element. I believe it's a parsing issue where cc-mode evaluates the following lines:

[foo
bar:^(BOOL *stop) {}
baz:

As 'statement, 'objc-method-call-cont, and 'label respectively, after a colon is inserted in line 3.

If there's any more information I can provide, please let me know.

-
Josh Vera