The following quote comes from an interesting series of essays from Reformed Protestant authors written in defense of the doctrine of "Original Sin." Notwithstanding the Reformed theology of the essays, the authors inevitably slip out of their (inconsistent) soteriology at times, evidenced from the following quote on the moral righteousness of OT figures:
Noah’s righteousness and blamelessness are not due to his utter sinlessness . . . When David, for instance, speaks of being “blameless” in Psalm 19:13, he does not refer to sinless perfection but to being “innocent of great transgression.” In the previous verse David asks to be declared “innocent from hidden faults,” so he is clearly not speaking of sinless perfection (Ps. 19:12). Similarly, David says that Yahweh dealt with him according to his “righteousness” in Psalm 18:20, but in view of David’s clear acknowledgment of his own sin (Ps. 51), we know that David is not claiming entire perfection when he speaks of himself as righteous. Thus, the description of Noah as “a righteous man, blameless in his generation” in Genesis 6:9 does not require the conclusion that Noah knew no sin . . . We can interpret them in keeping with the drift of the narrative, understanding that Noah was a winner like everyone else, and that Noah found favour in a way similar to the way that Abram later found favour.
Source: James M. Hamilton, “Original Sin in Biblical Theology,” in Adam, The
Fall, and Original Sin: Theological, Biblical, and Scientific Perspectives,
ed. Hans Madueme and Michael Reeves (Grand Rapids, Mich.: Baker Books, 2014),
pp.189-208, here, pp.196-97; cf. my previous post on a related issue.