Only use blacks in winter when targeting unicorns. Have used both fresh and frozen blacks , fresh if available where I’m fishing, e.g Dungy coast or Chesil and have had good and bad sessions on both. One worm on a hook ain’t gonna pull in the fish like 6 or 7 will, so using both I’ll fish fairly large baits with lots of scent.
If the fish ain’t there you won’t catch them, but a big juicy bait will give you more chance.
Using frozen crab left from earlier in the year has always been the only bait that I can notice a difference with in catch rate when after unicorns in winter on chesil or Kent way, and a couple times locally.