"Greater than great houses are the ships of the Go-hilleg, and they bear store of men and goods, and yet are wafted by the winds; for the Sea-men spread great cloths like wings to catch the airs, and bind them to tall poles like trees of the forest. Thus they will come to the shore, where there is shelter, or as nigh as they may; and then they will send forth smaller boats laden with goods, and strange things both beautiful and useful such as our folk covet. These they will sell to us for small price, or give as gifts, feigning friendship, and pity for our need; and they will dwell a while, and spy out the land and the numbers of the folk, and then go. And if they do not return, men should be thankful. For if they come again it is in another guise. In greater numbers they come then: two ships or more together, stuffed with Men and not goods, and ever one of the accursed ships hath black wings. For that is the Ship of the Dark, and in it they bear away evil booty, captives packed like beasts, the fairest women and children, or young men unblemished, and that is their end. Some say they are eaten for meat; and others that they are slain with torment on the black stones in the worship of the Dark. Both maybe are true. The foul wings of the Sea-men have not been seen in these waters for many a year. . ." A Certain Author
So I read this passage, as I read the
Moria Balrog passage, and indeed any other passage quite simply: I look at what I think the author is telling me. Really simple.
Now this passage contains multiple uses of simile/metaphor (I can never remember the exact terminology) where
Tolkien is obviously using the word 'wings' for 'sails'; nicely done, very poetic, no problems here at all.
Context is king here, as everywhere else; he introduces the simile, then reinforces it more than once. This allows him to end up with us knowing that the 'foul wings' of the Sea-men don't refer to actual wings but to the sails of their ships. Extended simile at its best.
What about the fact that he refers to the ships as 'greater than houses'? Are we always meant to have this image in mind, as we are about the sails? Well no, as he only mentions it once. That simile belongs to the first sentence alone. After that the ships are simply that - ships, not ships greater than great houses.
So the Moria Balrog can indeed have shadow-like wings or wings-of shadow in one sentence but this doesn't mean that in any following sentence the wings cannot actually just be wings.
Are the fairest women and children always to be seen as beasts? Is that what is eaten or sacrificed later, animal meat? Of course not, the simile describes their situation on what we could call a slave-ship, and later on we know that it is men women and children who will suffer.
Simile in its place, plain descriptive sentences also in its place.