The main difference I see is Dark provides a language, an editor and a runtime with a database - all integrated so e.g. the editor can display runtime and database info. It is also hosted so you don't have to run the servers.
Unison is mainly a language and runtime, you use any text editor and some unison tooling but code is stored in the file system. I'm not sure how you actually operate the runtime. Do you run it on one machine or multiple? How do you point it to the code you want to run, etc. - I believe this stuff is still not fleshed out. I also don't think it has a database, but I'm not sure.