Give this a whirl...if it crashes your session, try running..close the drawing saving changes, then reopen and your block should be ok.
(vla-put-name
(vla-item (vla-get-blocks
(vla-get-activedocument
(vlax-get-acad-object)
)
)
(vla-get-effectivename
(vlax-ename->vla-object (car (entsel)))
)
)
"mynewblock"
)