Here's what I drafted in my next draft. None of the red or blue cards I have seem so attractive as to make me play the color instead of the other. Final Flare might not be worth it, I thought I have things to sac to it (Woe Strider's token, last chapter trigger on Tymaret Calls the Dead, maybe even Alirios's token), but the power level on that card doesn't seem that strong even if I had something to sacrifice. 3 black devotion payoffs seems quite good, maybe I should run Grim Physician (this is another thing to sac to Final Flare I suppose). No idea how good Enemy of Enlightenment is, but I do feel like I need some win condition. Overall though, the deck seems pretty mediocre in terms of power level. I got passed a P2P3 Banishing Light, as well as an awfully late Voracious Typhon, but it felt too late to switch by then. Didn't open any powerful bombs either, sadly.
It's possible I'm overrating Catoblepas. Like, it's "almost" a Ravenous Chupacabra right? If I see Catoblepas fairly late, is it correct to say Black is open?
Final Flare is a good card when you're in red. You don't even have to sacrifice a valuable creature - any enchantment is fine. So best case scenario you sac
Omen of the Dead or
Iroas's Blessing to it.
Now the next question is whether these two cards are splash-worthy or if you'd run them as your actual support color with
Impending Doom and
Skophos Maze-Warden. Those are both good when you're in red.
Your blue cards are similar -
Witness of Tomorrow is
Cloudreader Sphinx or
Watcher in the Mist. Well, not exactly, it is slightly weaker because the scrying has a cost, but this is actually a common that you "never" see in TBH draft in my experience. It's a big flier that kills and survives most other fliers.
I'm also a big fan of
Alirios, Enraptured, which I've seen more often than the Sphinx. So much stats for 3 mana.
One with the Stars is mediocre removal. You can run it if you run Islands anyway. It's maybe even better than "destroy" removal for there's more graveyard interaction than with this enchantment.
Towering-Wave Mystic just stinks, is there a reason you have him in your deck? I'd rather run an
Eidolon of Philosophy or
Grim Physician. Eidolon is pretty great in the late game - obviously.
Starlit Mantle and
Shoal Kraken are solid playables, too. The 4 mana counter isn't very good, the other one is fine.
Now with all that being said, I'd probably go with your plan of going mainly black splashing the best cards of red and blue instead of going full Dimir. 9 Swamps, 4 of each of the others, wait with the amulet until you know which color is missing.
For the black cards:
Yes,
Blight-Breath Catoblepas can be a
Ravenous Chupacabra and this time you have a lot of black devotion (only reason to run the 4/3 4-drop over it's red cousin) so that this dream can come true. You're probably right though in that you're overrating him. It's 2 more mana and oftentimes a weaker effect than the green 4/4 that fights. The real stars at common in black are the two removal spells.
I'm not a big fan of the black omen, but it's okay filler. Could say the same for
Temple Thief,
Scavenging Harpy and
Tymaret Calls the Dead (despite it's rarity). Tymaret himself is always good and often a one-man cemetery keeper - your opp will never escape anything or use premium removal on a 2-drop.
Aspect of Lamprey is often a good card, but don't overrate it. 4 mana discard 2 and you need a creature that doesn't die during the application. It can help you make
Enemy of Enlightenment relevant, which he often isn't. Don't cast him when opp has 3 or more cards in hand. Your sphinx is a lot better, also your Wings can make anyone fly.
edit: The saga should be better than I gave it credit, but
History of Benalia this is not. Just 2 2/2s for 3, which is a good rate but worse than Alirios for example.