It is a metaphor, as an Allusion is an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference. I haven't read the majority of the letter, so I'm not exactly able to tell you what it is. I believe it is a metaphor because I think he is referring to the pain of African Americans in their oppression at the time, almost over flowing with an ugly emotion. If you dont think I answered correctly, you should probably talk to about it with your teacher, they should be happy to help you. You can also message me if you need some clarification on what I think. :)
