Block mark and line mark are somewhat different. Use line mark for moving, copying or deleting one or more whole lines and block mark for moving, copying, deleting, filling or overlaying columns of text. Note also that a block marked region can also be moved to the left with ctrl ∧ and to the right with ctrl _ but the behaviour is different than if the ALT arrow keys are used. ALT left and ALT right (or ctrl _ and ctrl ∧ ) are meaningless for line marked regions.
Block Manipulation Keys
ALT B mark column block start, end
ALT C copy marked block
ALT D delete marked block
ALT F fill B marked block with char
ALT L mark LINE(s) start, end
ALT M move marked block
ALT O overlay B marked block
ALT U unmark block
ALT Y yank back deleted block
CTRL_ shift B marked block right
CTRL∧ shift B marked block left