Why is it "Dw i wedi blino heno" and nit "Dw i'n wedi blino heno"? Isn't "Dw i'n" I am?
No; "I am" is dw i.
The yn ('n after vowels) is used to link a predicate (verb, noun, adjective) to this dw i to form the present tense, but here we have wedi instead to form the present perfect.
If you have wedi, you don't use yn.