I assume that blocking applies to O Linemen and that Footwork applies to defensive players?

I suppose if you want to improve an OL in Blocking that you focus on that and same if you want to improve a defenders Footwork, even though those attributes are listed as defensive and offensive categories respectively?

It seems odd that Agility is in both categories but not the above 2.

PHI 21 wrote: In my experience, OL need both blocking and footwork. DL need tackling and agility.

That's my experience as well.
My team is trained like that at OL and DL.