If Noah was a descendant of Cain, but turned good, what happened to the bloodline of Noah? How did Cain’s bloodline continue after Noah?

How is Noah related to Seth?
Genesis refers to Seth as the ancestor of Noah and hence the father of all mankind, all other humans having perished in the Great Flood. Seth is seen by Eve as a replacement given by God for Abel, whom Cain had slain.

Who was Noah the descendant of?
Biblical genealogy

Lamech is the eighth-generation descendant of Adam (Genesis 5:25), the son of Methuselah, and the father of Noah (Genesis 5:29), in the genealogy of Seth in Genesis 5.

I don't believe he was Cain's descendant. At least not according to what I've read....but then I don't even know if the story about the "missing" books of the Bible is really true. Maybe there's more to the story. But...going by the information I HAVE...it's my understanding he's Seth's descendant.

Noah is Seth's father

I think you might be thinking of Shem?

Seth
According to Genesis 4:25, Seth was born after Abel's murder by Cain, and Eve believed that God had appointed him as a replacement for Abel.

Noah
Noah features as the tenth and last of the pre-Flood patriarchs in the traditions of Abrahamic religions. His story appears in the Hebrew Bible, the Quran and Baha'i writings. Noah is referenced in various other books of the Bible, including the New Testament, and in associated deuterocanonical books. Wikipedia
Grandchildren: Cush, Aram, Canaan, Magog, Keturah, Lud, Put, MORE
Grandparents: Methuselah, Barakil, Edna
Great grandchildren: Nimrod, Midian, son of Abraham, Ishbak, MORE
Great-grandparents: Enoch, Azrial, Edna
Children: Shem, Ham, Japheth, Aram