It should be accepted, you can use the "report a problem" feature if you encounter this exercise again and your answer is still not accepted.
No, only one darkness, mais plusieurs ténèbres.
You can expect different languages to have differences. Even if they seem illogical to you, just consider them as if you were born with those, as you do with your native language. Everything makes much more sense after that.
By the way, "ténèbres" is always plural. If you really want a synonym, you can use "obscurité".