Id generally recommend avoiding black suits as they are more difficult to match than charcoal grey or navy suits and dont work well with many mens skin tones
The latter has never IMHO been a hard and fast rule (bearing in mind the varation in rise of trousers) nor does a minor transgression stand out in the way that a visible back blade does
(When you're a portrait painter, there's little chance you'll be impeached for perjury.) But Robert Langdon's fever dreams aside, there's a long tradition of artists hiding symbols and cryptic messages in works commissioned by the rich and famoussometimes with the commissioner's collusion, and sometimes not
I think cocktail attire is really nice and a great excuse to wear good tailoring, but perhaps in some ways a little less creative and open than black tie, just in terms of the colours, materials, different traditions and so on
For guidance on trouser pairings, see our guide on what pants to wear with a blazer
Couldn't recommend highly enough