I think I’ve just invented (unvented?) method to create horizontal cable border. Not thoroughly tested yet – tried it with 5 sts for a 4 st cable, but I think it would look better with some borders. Like this:

Finish RS row. Use Cable CO to cast on 10 sts.

R1:On WS, K4, P4, K1, K2tog.
R2: P2, K4, P2, K2.
R3: same as R1.
R4: P2, cable 4, P2, K2.

This gives a 4-st cable with 2-st purl border on either side and a 2-st garter border to help tame curl.

What I actually wanted, though, was a horizontal cable that still had live stitches on the far side, so it could be a band instead of a border. Will have to consider further if there’s a way to do this (other than just picking up stitches on the edge of the border).

ETA: Actually I ended up casting on 29 stitches, using the same cable pattern as in the rest of the blanket with the orignial end st making 30 – 2 st reverse stockinette border, 24-st cables, 2-st rev stockinette, 2-st garter. Slip the 1st st on every WS row for a neater edge, and k3tog instead of k2tog at the end of every third WS row to make the border even with the main blanket.