I actually thought that Joel was denied on purpose. The point being to create a new way of fighting: With a rithmatist and a non-rithmatist in the same circle. So the Shadowblaze showed up to tell him he's special, but did not bind because Joel has another purpose. For all we know, the way to ultimately defeat an uber-forgotten is exactly though this kind of pairing. Nalizar never saw it before because it was never done before.
Of course, it could also be because there weren't any free spots available (as rithmatists weren't dying). In which case, the chalkling is not a shadowblaze at all, but a chalkling sent by someone/something else. But this seems sooo much less cool.