Do you change vilkett to vilka and blått to blåa when going from singular to plural?
Not every word, but generally the number (sing./pl.) and a grammatical gender (en/ett) will affect words describing the object.
bluebirds are blue. Peacock. There is also Blue poison dart frog which is blue. Blue parrotfish. Some butterflies and dragonflies, but those are bugs and blue dragon snail: Glaucus atlanticus which looks awesome.
There is a blue bird (your said bluebird?) Which also collects blue things, some geckoes, there was an agama (lizard) with a blue head, ants, wasps, chameleons I guess can be blue... ah, and everyone should google this snail Jacob mentioned. Really amazing indeed.
Given we're learning Swedish we shouldn't forget Svensk blå anka https://sv.wikipedia.org/wiki/Svensk_bl%C3%A5_anka
really, that's a question for whether they have cell membranes or cell walls. I propose a smurf hunt to find this out.
It's ett djur and ett things are blått. (en words and plural can be either blå or blåa).
I think it would be Vilken; Vilken
hundarhund, vilket djur (which animal), vilka djur (which animals). I think this is the only question-word to have en/ett/plural forms. Hopefully someone will correct me if I'm wrong!
Djur is an ett-word. VIlken is for en-words.
Vilket djur är blått?
Vilken hund är blå?
What are the dots (ä) and the circle (å) called? And is there a name for a mark above a letter like that?
The overarching term for marks like these is 'diacritics'. This term refers to all kinds of glyphs added to letters, including in-letter ones (like in 'ø') and sub-letter ones (like in 'ç').'
The dots above the 'ä' are called an 'umlaut'.
The circle above the 'å' is called an 'overring' or just a 'ring'.