Lord Rama was blue-black.
Krishna was Black.
The term Krishna means Black.
Both Rama and Krishna are the Avatars of Vishnu.
Then why the difference in skin tone?
Vishnu appears as,in
White, with four arms,with hair pulled up(Jatamakuta),
Wearing Maravuri, clothes made of barks,
with Akshamala Kamandalu and Dhanda.
With Vedas as Four Arms.
Bare arms and Arrows.
Source.Bhagavatha Purana Chapter 30.