1- We have narrations that state Imam al-Jawad (a) was dark skinned. Some narrations indicate that Imam al-Redha (a) was also dark skinned.
2-Some historical accounts state that the enemies of Imam al-Redha (a) made racist remarks against him for being dark, and they would mock him. Sometimes they would even write a poem trying to disgrace the Imam for being dark. For instance, this is a racist line of poetry they said to the Imam:
وقالوا انه رب قدير *** فكم لصق السواد به لصوقا
3- Yes, a number of Imams married dark skinned women, some of whom were taken as slaves. This includes Imams al-Redha, Jawad, Hadi and Askari, peace be upon them.