I can't resist a golf!

    ruby -e 'print %W{\u2571 \u2572}.sample while sleep 1e-3'

(Updated: Made 3 chars shorter.)

And a shorter, but cheats, version:

    yes|ruby -pe'$_=%W{\u2571 \u2572}.sample;sleep 1e-3'

My favorite overall balance of readability and shortness though is:

    ruby -e 'print %w{╱ ╲}.sample while sleep 0.01'