Here's how to understand it:
* Relative pronouns connect a dependent clause (a clause that can't stand alone) to an independent clause. They also act as the subject or object within the dependent clause.
* "Whom" is used as the object of a preposition ("after") and it introduces the dependent clause "after whom your friend named".
* The sentence is referring to William Blake, and the dependent clause tells us something about him.
Let me know if you'd like to see another example!