The pin that blocks the blade hole when pliers are open is probably deformed. You can try pushing the pin back into it's nesting area when the whole tool is closed, back into the position it's supposed to be in that state (which allows the blade to open freely). However, I have a feeling that the pin (which is riveted inside the handle and is actuated by the ramped cam action on the plier head) may be fatigued, and may not survive too much bending. In which case, it will snap off. Which isn't really a big deal, functionally.
The alternative is to send it back to Leatherman, explaining the problem.