A Vaishnava-based answer ahead…
Actually, if you see, the color of Lord Krishna doesn’t depend on what color Devaki Mata and Vasudeva have. Because, if you see in human biology, and in that too hereditary, then the child inherits the genes of the mother and father both. But here, Krishna is not born like a mere human being. As the Bhagavata Purana itself calls Him unborn, who was about to enter the womb of Devaki. And another thing is, According to the Bhagavata Purana, Anantashesha, with the avesha of Sankarshana roopi Bhagavan (who later appeared as Balarama) was first present in Devaki's womb for 7 months and was then transferred by Yogamaya Durgadevi into Rohini's womb. After that, Lord Vishnu entered Devaki’s garbha and lived in her garbha for 3 months, after which He came out of her garbha and appeared as Krishna.
While coming to, Krishna didn’t come as a normal baby as He came in Ramavatara. He came out as a divine child wearing a crown, studded with jewels, and holding conch, discus, mace, and lotus in His four hands, with pitambara, Srivatsa mark, and all. Krishna was not born through ordinary biological processes like an ordinary human being. He manifested from the womb of Devaki by His own divine will. He appeared as the son of Devaki and Vasudeva, or more precisely, appeared from Devaki’s garbha rather than being born from Devaki’s garbha. Plus, He is Bhagavan. Since His appearance is divine rather than the result of ordinary material birth, biological inheritance cannot be treated as the determining factor for His complexion. He is always transcendental, be it as Lord Vishnu in Vaikuntha, or Lord Krishna or Lord Rama in the Bhuvaikuntha, Vrindavana/Dwaraka or Ayodhya.
SriKrishna in the BhagavadGita says:
अजोऽपि सन्नव्ययात्मा भूतानामीश्वरोऽपि सन् ।
प्रकृतिं स्वामधिष्ठाय सम्भवाम्यात्ममायया ॥ 6॥
Although I am unborn, the Lord of all living entities, and have an imperishable nature, yet I appear in this world by virtue of Yogmaya, My divine power.
[Bhagavad Gita: Chapter 4, Verse 6]
So, why was Krishna black in color? His color is described as Neelameghashyaama, which means the color of rain-filled clouds. This color is not pure black, but bluish-black. The reason why He is black is different. In every yuga, He has different colors. In Krutayuga, it was Shuklavarna Narayana, which means the Lord was white colored. Shukla indicates knowledge, so for gyaana karya, He had Shuklavarna or white color. In Treta yuga, He was Raktavarna, which is a red-colored form. In Dvaparayuga, He came in Krishnavarna. Why Krishnavarna? To bless His dear devotees. Even Gargacharya, in SrimadBhagavatam, during the naming ceremony of Krishna and Balarama reveals Nanda Maharaja, that Krishna is none other than Lord Vishnu Himself. Even earlier He has assumed white(shukla), red(rakta) and yellow(pita) forms, and now He is present in His dark-neelameghavarna form (Krishnavarna).
[Shuklavarna Narayana]
[Raktavarna Narayana]
[Peetavarna Narayana (yellow)]
[Krishna varna Narayana]
Just to give a rough idea of the colors, I have inserted the above images. So, the dark color of Krishna is not related at all to the color of Devaki and Vasudeva. He is transcendental every time and can assume any form and any color. Didn’t He appear as Narasimha from the pillar? Srimad Bhagavata describes Him as of golden color. His beard, His face, everything was golden in color.
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!