The Blackest of Hearts by Emma V. Leech

Lord Blackehart has not a drop of blue blood in his body, but he is the undeniable Lord of London’s underworld.
Born into poverty and quickly abandoned, he is left to grow up in the misery of the workhouse. Rumours about him are many, that he cannot die, and that he killed a man when he was just twelve years old. Now a man in his prime he is built like a mountain and exudes power.
Katherine Dove is illegitimate. She has borne the shame of this her whole life, but when her guardian and her only protection from the world at large dies, she discovers a clue to her parentage.
A perilous journey to London in search of her father finds her in dark company, but Miss Dove is resourceful, and Luther Blackehart seems to be a man who can supply the answers to everything and anything she desires.