What this Enchantment does is both simple and powerful. Each time a creature dies, no matter who owns it, you put a charge counter on Black Market. For each of those charge counters, you get a Black mana during each of your precombat main phases. Hurray for lots of free mana!
With its first reprint, some players were surprised that Wizards of the Coast decided to print a card of this power level in a pre-constructed deck. Others didn’t feel like the synergy made sense in a White/Black deck like Call the Spirits, although the deck cares about Enchantments. Nevertheless, Black Market is a great card to have available for newer Commander players, and Wizards even reprinted it again for good measure.
All you need for Black Market to become ridiculous is have a bunch of creatures die on a regular basis. Why did Wizards ever print such a crazy way to gain nearly free mana? While this card seems pretty busted now, it wasn’t always so. When Black Market was first printed, the mana burn rule existed. This rule stated that any mana you didn't use during any phase of a turn would cost you 1 life for each mana that goes unused when changing phases.
For example, if you had 5 black mana still in your mana pool when switching into your combat phase, you would lose 5 life. This meant that you had to use that mana right away or take damage. This rule was removed from Magic around the release of the Magic 2010 Core set, which made cards that produced mana that could end up burning you considerably better.
Braid of Fire is another card in this vein, which can produce an unholy amount of red mana. Due to its cumulative upkeep, it produces an additional red mana with each upkeep that it remains on the field; this is mana that Commander decks have no trouble using at instant speed. It's nowhere as popular as Black Market, due to the raw power that mono-Black has in the format. So why include it an entry-level Commander product not just once, but twice?
The only answer I have to that question is that Wizards knows that players love playing mono-Black. Why else would they continue to power up the color? In any case, the White/Black deck does have ways to take advantage of this extra mana, but not in the way that mono-Black does. The list of Commanders that can take advantage of this card is huge; it’s far too long to list here. Black Market is definitely a chase card in both Commander decks in which it’s reprinted. Having so many copies enter the market has been really good for the format by getting a powerful card that was once fairly rare into the hands of players who wanted it.