At all times any reference to someone's righteousness must be either a relative one, or one as a result of justification (the imputation of fulfilled justice as a result of God's mercy on the sinner). This is due to:
Romans 3:9 What then? Are we better than they? Not at all. For we have previously charged both Jews and Greeks that they are all under sin.
10 As it is written:
“There is none righteous, no, not one; 11 There is none who
understands; There is none who seeks after God. 12 They have all
turned aside; They have together become unprofitable; There is none
who does good, no, not one.”
Which is in turn a reference to verses such as:
Psalm 143:2 Enter not into judgment with your servant, for no one living is righteous before you.
1 Kings 8:46 “When they sin against You (for there is no one who does not sin), and You become angry with them and deliver them to the enemy, and they take them captive to the land of the enemy, far or near;
Job 25:4 How then can man be righteous before God?
Or how can he be pure who is born of a woman?
Psalm 14:3 They have all turned aside,
They have together become corrupt;
There is none who does good,
No, not one.
Ecclesiastes 7:20 For there is not a just man on earth who does good
And does not sin.
and is reinforced with:
1 John 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
All men that were counted as righteous in the Tanach (Old Testament) were not as a result of their "goodness" or good behavior (as can be deduced from the above verses), but rather as the mercy of God being extended to them, as we can see in Genesis 15:6
Genesis 15:6 And he (Abram) believed in the LORD; and he counted it to
him for righteousness.
Further, (and to lean a little more on theology) this is only as a result of God imputing the righteousness of Christ (who never sinned, as per 1 Peter 2:22) to all who trusted in the mercy of God, and yes, even for Old Testament believers. This is what the title of "the Lamb slain from the foundation of the world" is referring to.
Revelation 13:8 And all that dwell upon the earth shall worship him,
whose names are not written in the book of life of the Lamb slain from
the foundation of the world.
His righteousness is "applied", even retroactively, to those who lived before Him, including Noah. So Noah's righteousness, like that of Abra(ha)m's, was as a result of His trusting in the Mercy of God alone, and not their own righteous deeds. So to answer your question, Noah was relatively good, sure, but not blameless in his own behavior, he was blameless because God had already had his sins paid for by the Messiah who was to come (cf. Isaiah 53). This imputed righteousness is what allows God to treat us as righteous without his own righteousness (justice) being violated. Thus, he could find Noah to be "blameless" in the absolute sense, even if he wasn't blameless even in the relative sense.
