While elisp files are correctly highlighted, there is no established short name for the language. I think it's just a matter of adding another short name to the CommonLispLexer (and I may dig into the code and put together a pull request to that effect if I find myself motivated).
I only noticed this because I have some Org files on Github with elisp code blocks that are not highlighted (while, say, CL and Clojure blocks are).
If someone beats me to it, I propose
emacs-lisp as the short name in order to play nicely with