Maverick is very well known to traditionally be a GW midrange deck, however 75’s have always popped up with a splash of black for access to some huge gains in sideboard options.
When building your 75 you need to consider what the true weaknesses of a fair deck like Maverick are. In my opinion, combo (especially fast combo) decks and any archetypes that can go wide are traditionally poor match ups for Maverick. Miracles also comes to mind as unlike another creature based deck like Death & Taxes, you don’t have access to Aether Vial to support your ability to reload the battlefield at instant speed with threats after a boardwipe like Terminus or Supreme Verdict.
In my opinion the two main cards that warrant the black splash in Maverick are Thoughtseize and Zealous Persecution, with the luxury of also being able to play one or two Abrupt Decay or Assassin’s Trophy in your 75.
Thoughtseize is one of the most powerful cards in Magic and is a great way to interact against Control and Combo match ups such as Miracles and Storm. Not only do you get to know the contents of their hand, you can cripple it by taking the best card they currently have access to or even the sole reason they kept. Thoughtseize can prevent fast combo by taking a card like LED Infernal Tutor or even win conditions like Ad Nauseam or Empty the Warrens. Hand disruption gives you two powerful things, knowledge and choice. or take a card that’s going to hinder your plan like Swords to Plowshares or Terminus.
Zealous Persecution is a one sided board wipe that can blow your opponents battlefield to oblivion. Great against creature based decks like Death & Taxes, Goblins, Elves and even Pyromancer strategies, this card also has a sole purpose to combat True Name Nemesis.
True Name Nemesis is a big reason why Maverick need access to a hate card like Zealous Persecution. With GW builds you do have access to cards like Council’s Judgment or Blessed Alliance, but Zealous Persecution in my opinion is fair superior when looking to deal with creatures. Zealous Persecution is also a great answer to Goblins created from an Empty the Warrens and can warrant a concession even before resolving.
In tight situations, Zealous Persecution can also save your creatures from an opposing Zealous Persecution, Golgari Charm, Marsh Casualties, Toxic Deluge or Massacre.
The black splash also gives you access to Abrupt Decay and it’s newest comparison Assassin’s Trophy. There’s still some debate over which is better – the accountability of Decay or the versatility of Trophy. Currently I’m a big fan of Trophy online as having a card that is able to disrupt my opponents mana base against decks that don’t run basics or get myself out of a Dark Depths situation and also cover as an out against Planeswalkers like Jace, the Mind Sculptor or Gideon, Ally of Zendikar is too good to pass up.
In the end it really comes down to the meta you play most in. Online, I love the access to these cards as I feel it makes my bad matchups better. With access to a local legacy community, you can see which decks you’ll most likely play against most often and build your 75 from there.