Yes, it is. This is because of Esperanto's free word order.
Both "nigran libron" and "libron nigran" are possible (although the first is much more common), and "virino libron legas", "libron legas virino", "libron virino legas", etc. are all possible and all mean "a woman reads a book" (but with different emphasis).
Combining these two facts, you can make a sentence like "virino nigran libron legas" (a woman reads a black book), where the meaning changes if you remove the -n of nigran: "virino ❤❤❤❤❤, libron legas" (a black woman reads a book)
And by the way, despite the unfortunate similarity to a certain N-word in the US, "❤❤❤❤❤" (black) and "nigrulo" (black person) are in Esperanto generally considered to be neutral, non-offensive ways to refer to the race of a dark-skinned person with African ancestry.
Adjectives get the -n accusative when they are part of a noun phrase that is either the direct object of a transitive verb or the object of a preposition when there is motion. Adjectives must agree with nouns in number and case.
La libro estas ❤❤❤❤❤ -- No accusative, not a direct object.
Ŝi legas libron/Ŝi legas nigran libron.
She reads a book/She reads a black book.
The book is the direct object. What is being read?
Li saltas sur la tablo/Li saltas sur la granda tablo.
He jumps on the table/He jumps on the big table.
Li saltas sur la tablon/Li saltas sur la grandan tablon.
He jumps onto the table/He jumps onto the big table.