Why is the line from Seth to Noah often called righteous?

The line of descendants from Seth to Noah is often called righteous, in comparison to Cain's descendants who are called evil. This comes up when discussing Methuselah who although he died in the year of the flood supposedly can't have died in the flood because he was righteous, and as an explanation of what it means when the 'sons of God' took the 'daughters of men' in Genesis 6. But Genesis 6:9 says Noah was the only blameless person alive! Is there any evidence in the Bible to say that the rest of his ancestors were righteous rather than just being sinful like everyone else? (Excluding Enoch of course.)