[Gen 6:9]:

"This is the account of Noah. Noah was a righteous man, blameless among the people of his time, and he walked with God."

Hebrews 11:7 sheds some light on the subject of Noah's righteousness - that it came about through the faith of Noah and not his actions. His actions instead reflected the fact that he had placed his faith in God:

[Heb 11:7]:

"By faith Noah, when warned about things not yet seen, in holy fear built an ark to save his family. By his faith he condemned the world and became heir of the righteousness that comes by faith."

Acknowledgment to God alone of sin is the only way that any man, (all of whom are born in sin, ((Ro 5:12)), can deal with his sin before God thus to be accounted by God as blameless. This acknowledgment to God first comes by faith in God's plan of salvation to deal with that sin by His grace alone which is through trusting alone the sacrifice of His Son for that sin:

[Eph 2:8-9]:

(v. 8) "For it is by grace you have been saved, through faith - and this [salvation is] not from yourselves, it is the gift of God -

(v. 9) not by works, so that no one can boast."

[Compare Ro 4:5]:

"However, to the man who does not work but trusts God Who justifies the wicked, his faith is credited as righteousness"

Compare the full study on this matter

After one has accepted Christ as Savior from one's sins - settling the matter of one's sins in eternal matters, i.e., being declared blameless unto eternal life, then the acknowledgement is to be to God relative to one's daily sins in order to be declared righteous and purified, i.e., blameless on a moment to moment basis in one's daily life:

[1 Jn 1:9]:

"If we confess ["homologomen" = acknowledge] our sins, He is faithful and just and will forgive us our sins and purify us from all unrighteousness."

Compare the full study on this matter

Therefore, we can conclude that Noah acknowledged his sin before God and God accounted him blameless as a result of that faith. Noah did this because he trusted that God would indeed take care of his wrong doing.

So Noah, Job, Abraham, David or anyone are not seen by God as blameless in their own right but by the right of Christ, the Son of God, Who died for that sin.