Premise for the research
From Wikipedia's article about Elves in fantasy fiction and games, I've compiled a list of possible candidates in which shadow-melding could be an elvish motif of magical nature:
- The King of Elfland's Daughter (1924).
- The Broken Sword (1954).
- Dungeons & Dragons.
- The Elder Scrolls.
Analysis of the Elves' Nature in the different Universes/Lores
Orion watched him go with his bow in his left hand, till he (Oth) disappeared in the wood,
like a shadow going to a gathering of shadows and merging amongst its fellows.
The King of Elfland's Daughter, Chapter XI: The Deep of the Woods.
The quote above is the only one that vaguely adresses the shadow-medling matter. Similarly to Tolkien's works, this seems to be a physical camouflage or rather just a figure of speech.
There seems to be no reference at all of any kind of invisibility in the shadows in The Broken Sword.
In Dungeons & Dargons there is an Illusion Spell called Cloak of Shadows that is described as follows:
Your body seems to shift and fade between the material plane and the plane of shadow.
In The Elder Scrolls (pre-Oblivion games, as Oblivion was launched a few years after Warcraft III) there is a similar skill called Chameleon:
This effect lets the target blend into the surroundings so no one can see him.
There also exists Shadow Walk (which is described exactly as D&D's Cloak of Shadows, but I'm not sure if it is featured before Oblivion).
The issue with these abilities (both of magical nature) is that they are class-specific abilities rather than race-specific. Consequently, in these two universes there are no Elves capable of shadow-melding as a general rule.
The final conclusion is that what I believed to be a trope in fantasy, is no trope whatsoever. Apparently, not only Warcraft III is the first to introduce Elves with the inherent magical ability to merge into the shadows, but it also seems to be the only (please, correct me if I am wrong) Universe in which Elves have this magical feature.