I don't know why people associate Tolkien with adolescence or black & white morality. I actually think his books describe the most complex & subtle moral perspectives of any literature I've read. Honestly, I think "gritty" under the pretense of being more morally realistic or complex tend to do exactly the opposite. Where naivety assumes everything is good, cynicism isn't any more sophisticated for assuming everything is tainted. Not that noir can't be entertaining, in a self-indulgent fashion.
But anyway, here's my summary of Tolkien's morality.
It's superficially very black & white, but if you look at the progression through middle-earth's history, you find that heroism is never rewarded. All the heroes of his stories meet tragic ends, and somehow inadvertently end up making the world a worse place. Even the gods of the world, the Valar, end up separating themselves from the world in the end, because they find that when they intervene to try and make things right, they only seem to make things worse. The seemingly infallible epic heroes of virtue are also always revealed to have common features with the "dark forces" of the world -- pride, covetousness, ambition, jealousy, etc. It is always these qualities that bring about the ironic end of any character in his works, whether good or evil. The only difference is that good will have these qualities in a well-intentioned, restrained, or structured and culturally accepted manner, where Morgoth, Sauron, etc will be completely made of them in the most selfish manner possible. "Evil" is always the instigator of conflict, but "good" is an equal and willing participant. Keep in mind that a single event set in motion the entire history of Tolkien's world, that is Feanor's creation of the Silmarils. He was so proud and possessive of them, which he had every right to be, as he created them. Morgoth falls in love with them and kills Feanor's father in the process of stealing them. Feanor then leads his people on a massive suicide mission to middle-earth to retrieve them and avenge his father. If this is black & white morality, then apparently you can turn any dispute into black & white with the words "he started it!"
There are only two ultimate forces of good in Tolkien's work -- Valinor and the hobbits. Ironically, they're not actually "good" so much as very very neutral. Both Valinor and the shire lead very simplistic and egalitarian lifestyles. The only major difference is one is inhabited by gods, and the other by unimpressive midgets. Both mostly ignore the conflicts of the outside world, finding them foolish, but stepping in when things get so out of hand that it starts spilling out of bounds. This is why the hobbits were so resilient to the corruption of the ring. It corrupted people through their natural ambitions. Hobbits had none. Simple people. Frodo and Sam didn't want to be heroes. Their culture even frowned on the very concept. They were only doing what they found needed to be done. This is what Tolkien presents as an absolute ideal.
At the same time, he concedes the necessity for "good" lest the world be overrun by "evil", but with the warning that they carry the same corruptible motivations. He portrays the likes of Boromir, Aragorn, Galadriel, etc in a very positive, but very dangerous light. But this is also related to one of his other core lessons. The landscape of heroes and villains is what makes the world an interesting place. This is actually the first moral lesson in all of his works. As the Valar are first creating the world, Morgoth tries to oppose them in everything they do. As this happens, Eru Illuvatar points out to them how his opposition enriches the world. From Wikipedia, as I don't have a copy of the Silmarillion on hand at work "Melkor’s attempts to disrupt with the use of fierce heat and severe cold do nothing to ruin Water (as Melkor must have hoped), but rather leave the World with the beauties of snow and frost and clouds and rain." This is a continuous theme throughout Tolkien's mythology, as great conflict is seen as both a negative and a positive. It always leads to both great beauty, great destruction, and epic tales that enrich the cultural and historical landscape of the world. The Valar even concede this to Feanor as they advise him against going to war against Morgoth, but admit that the events which transpire will surely make for great songs and stories. As time passes in Arda, the scales of conflict between good and evil continually diminish, and the nature of the world grows less and less colorful. In the end, all the immortal and powerful beings of the world end up leaving, and all that's left is grey and uninteresting. There's no great ugliness, but no great beauty in middle-earth either. It's very bittersweet.
And the final major point of morality in Tolkien's mythology is that good never triumphs over evil. Evil defeats itself. Good, while sharing common, corruptible qualities with evil, is separated by ultimately positive intentions and codes designed to uphold those intentions. Evil is not so restrained, and is much more alluring and thus more populated. The forces of good always always always lose, throughout the entire history of middle-earth. They win battles, but never wars. But the destructive nature of evil always turns on itself in the end. This is most obviously displayed in the destruction of the ring (where the movies failed most greivously). Sam and Frodo were not successful in destroying the ring. Gollum was. Or rather, the ring poisoned his mind to such a point of insane single-minded obsession, that he completely lost all awareness of his surroundings when he finally got his precious back and dances with joy right into the crack of doom. The will of Sauron carried out in the corrupting influence of the ring unwittingly orchestrated its own destruction. This was a huge lesson for me when I was younger, and I didn't encounter the same sentiment in any other work until I was much older.