In vim, there's a CtrlV to do block selection.
How can I do it in
Assuming you already know how to use Space to do normal selection in
screen's copy mode, the only new keys you need are c to set the left margin, and C to set the right margin.
This is described in the
Selection for copy: Ctrl+A [
Copy current selection (First to select start corner, second to copy selected area): Enter
Cancel current selection: Esc
Paste into screen: Ctrl+A ]