Providing a username by itself to bkusers, or with the userspace prefix, works just fine, but passing a username with the user talkspace prefix shuts down the whole query, e.g. https://en.wikipedia.org/w/api.php?action=query&format=json&list=blocks&bkusers=User%3AAmorymeltzer%7CJimbo%20Wales%7CUser%20talk%3AAmory

Ideally, the parameter would be able to handle usernames like User talk:Jimbo Wales but at the very least, I would think the query should continue if there are other, valid usernames given.