Tactician can be an annoying card. Playing it means forgoing your current turn for a doubly-good next turn. This is good when you have a bad hand, or a reduced hand, but not good when you draw the Tactician with all your Platinums.
Black Market provides an interesting (and devastatingly powerful) way out of this dilemma, as a hugely easy way to implement the Double Tactician engine.. Play Black Market and use it to play your Treasures. Whether or not you buy from the Black Market, your Treasures are now available for the subsequent Buy phase. So long as you have at least one additional card remaining in your hand to be discarded, you can now play the Tactician, have a normal Buy phase with your Treasures, and still have a mega-turn next turn.
Black Market essentially eliminates the primary drawback of Tactician and turns it into a juggernaut. Provided you can consistently draw the Black Market and Tactician, you can have 10-card hands every turn. And although it might seem that you need a +Actions card to be able to play Black Market and then Tactician afterwards, in practice, the extra Action from Tactician fills this need perfectly.
There are several other combos that make use of this Black Market ability: if you use Black Market to play a Quarry or Royal Seal, then the Treasure’s effect can be used on gaining Action cards. For example, playing a Quarry with Black Market lets you then play an Ironworks to gain an Action card costing up to $6. Even better, play a Royal Seal with Black Market, then Ironworks two Treasure Maps onto the top of your deck. But Black Market / Tactician is probably the most powerful combo, and easiest to pull off.
I was lucky to have drawn a Cellar and Bank from the Black Market, which made it a lot easier. The set was pretty convenient, since it contained Colonies/Platinums as well as a good $4 buy (Mining Villages).
A more archetypal example, with Crossroads support