This is not about race, is about culture. You can have any black character in middle earth, --any--, as long as is Southern. Nobody would bat an eye. You can made this character as heroic and positive as you want.
Having a black elf is a miscast, because most races in LOTR depict real groups of people or real cultures in Europe. Elves are tall and blonde and represent the Germanic and Scandinavian traditions that Tolkien loved. They are strongly linked to Alpine mountains and European forests. Their language is based in how Nordic languages sound.
An Asian actor could fit in the role as long as is tall and thin but a credible African elf is much more complex to construct. Can easily feel out of place and as miscasted as would be seeing a black actor in the role of a german nazi soldier.
Harry Potter has black actors, but all of them are from UK (very smart move JK) and share a common culture. Their interactions feel much more realistic and cohesive by that.
Just really cannot fathom this if you are serious. There are literal tree people and talking eagles and giant evil wizards in this world for godsakes! It is a fantasy world! Like truly, how can you care about this? Do you not feel any dissonance in your logic here? It is just beyond parody.
There is much more in the book than cheap amusement. It works in several different planes and there is a lot of philosophical and metaphorical content.
To write a fantasy book is easy; to do it at the Tolkien's way is extremely hard. His search for consistency in the entire work is legendary. All tiny details fall in the correct place. LOTR set the mold and reference for every single book in the "sword and sorcery" category for a good reason.
As video gamers have it - there's only two races: white and "political", only two genders: male and "political" and don't forget the only two sexual orientations: straight and "political".
1) A natural fit with strongly intuitive significance, or 2) a forced fit where representation is the highest goal at the expense of feeling distracting and nonsensical, even to those being represented.
