I apologize if this has been asked here.
It looks like people would rather purchase 5 or 10 or 20 different types of suits before they even considered buying a black suit. So my question is, when is it correct to wear a black suit? Are there any situations where is may not be preferred (over charcoal or something similar), but still acceptable?
If you are asking about the acceptability of black suits in the context of traditional men's suits, there are so few occasions for which they are acceptable that it would be silly to acquire one unless, as you mention above, you already own 20 or more other suits. If you are asking in the context of today's plastic, duck-billed shoes and glued polyester Men's Wearhouse suits, then black is acceptable and even preferred for lots of situations. Choose the color of your suit based on the standard to which you prefer to conform.
Since many people criticize the seemingly arbitrary character of the "rules" of traditional menswear, I will point out that charcoal suits look objectively better than black suits in the vast majority of shirt/tie combinations that would be worn in a day or business context. In other words, there are objective aesthetic bases for many of those "rules". Even the most casual observation of the duck-billed shoe/polyester black suit of today should illustrate the aesthetic carnage that results from discarding all those "rules" at once.
That's highly debatable. Charcoal is perfectly appropriate for funerals.