Does Randuins reduce shadowflame crits?

It does, but as with all crit damage modifiers on Shadowflame, it only applies to the bonus damage from the crit, not the original damage.

So if you deal 100 magic damage and it's amped 20% by Shadowflame to 120, Randuin's doesn't decrease the entire 120 magic damage, just the 20 bonus being added by Shadowflame. In this case Randuin's would reduce the damage to 114.