Black Pawn refers to the publishing company that publishes Richard Castle's books. Castle's ex-wife Gina Cowell is an employee of Black Pawn, thus in A Deadly Game (episode 2.24), refers to herself as Castle's publisher when she is introduced to Kate Beckett.
In the acknowledgments section of his first Nikki Heat book, Heat Wave, Castle "grudgingly" thanks Gina and the folks at Black Pawn for threatening legal action against him (Gina threatens to sue Castle to get back his advance on his next book in the series premiere Flowers For Your Grave) which gave him the impetus to write Heat Wave. Castle had experienced a bit of writer's block after killing off Derrick Storm, the main character in the very successful series of novels that he wrote before meeting Beckett and creating Nikki Heat. His meeting Beckett, combined with Gina's and Black Pawn's threat, got him writing again.