bled her with guns (16)
The funny thing is, it's not actually Africa who is bleeding; it's the people of Africa. Angelou's speaker uses the continent as a shorthand for all of its inhabitants here.
her history slain (23)
Slavery may be (almost) over, but the loss of history is something that continues to haunt colonized peoples the world over. Ripping people away from their cultures and communities makes it hard for them to maintain a sense of their past. In many ways, this is a form of violence just as important as physical violence.
remember her pain remember the losses her screams loud and vain (19-21)
Now suffering opens the door to transformative experiences as the speaker urges us (the readers) to use suffering as a rallying cry to fight for a better future.