You cannot add two blanks in a row, it won't work. The whole menu_addblank native seems flawed and you should use ^n in the previous item for a newline instead.
Also, this is completely unnecessary. MPROP_PERPAGE doesn't remove the exit button and while the .inc file says it's deprecated, it really isn't. menu_setprop( menu, MPROP_PERPAGE, 0 ) positions the Exit button at slot 0 and also removes the previous/next items IIRC.