Krishnā was born with a dark complexion. The word Krishnā means one that attracts. Another meaning is dark. Since Draupadi was born with a dark complexion and was extremely attractive, she was called Krishnā.
Then a young maiden arose from the centre of the altar. She was blessed with good fortune and was known as Panchali. She was beautiful and her waist was shaped like an altar. She was dark. Her eyes were like the petals of lotuses. Her hair was dark blue and curled. She was truly a goddess born in human form. The sweet fragrance of blue lotuses emanated from her body, a full miles away. Her form and supreme beauty were such that she had no equal on earth.
Chapter 155 - Chaitraratha Parva, Section 11, Mahabharat
It clearly states that Draupadi was born with a dark complexion. Her beauty was unequal in the whole world.
The Brahmanas, whose desire were entirely satisfied, gave the two names, “Because of his great courage and because he has been born from lustre, let this son of Drupada be called Dhristadyumna. Because she is dark in complexion, let her be called Krishnā.
Chapter 155 - Chaitraratha Parva, Section 11, Mahabharat
For a similar reason, Arjuna was also called Krishnā as he was of dark complexion and handsome. Before the Virata war, while retrieving the weapons, Arjuna revealed to Uttara his ten names and their meanings, one of which was Krishna.
Affectionately, my father gave me my tenth name, Krishna, since the child had a dark complexion.
Chapter 635 (39) - Go Grahana Parva, Section 47, Mahabharat
Thus, Krishnā was the actual name of Draupadi and was so named because she was all attractive and dark-complexioned.
