I would advise you to visit your paediatrician at the earliest. Some newborns are born with an undetected heart problem. The dark colour might be due to cyanosis
which suggests defecient oxygenation. What is the colour of baby's nails and fingertips? Is your baby sucking well at the breast?Please visit your paediatrician at the EARLIEST to rule out heart problems/ other conditions.
Have a nice day :)