Genesis 6:9 says that Noah was perfect in his generations. 

Gen 6:9  This is the genealogy of Noah. Noah was a just man, perfect in his generations. Noah walked with God.

Some people understand this to mean that Noah was racially pure, that is there were no mixed marriages in his ancestry.  Is that what this verse really means?  Let’s take a closer look.

Gen 6:9  This is the genealogy [toledah] of Noah. Noah was a just [tsaddiyq] man, perfect [tamiym] in his generations [dor]. Noah walked with God.

The Hebrew word translated “genealogy” is toledah, which means descent, family history, generations, genealogies.  

The Hebrew word translated “just” is from the Hebrew word tsaddiyq, which means just, lawful, righteous.

The Hebrew word translated “perfect” is from the Hebrew word tamiym, which means complete, whole, entire, upright, without blemish.

The Hebrew word translated “generations” is from the Hebrew word dor, which means period, age, generation (a period of time).

From the translations of these Hebrew words we can see that Genesis 6:9 is not making a statement about Noah’s bloodline, but rather about his character in contrast to those around him.  Noah was a righteous man.  If we look at the context, we see that the world was filled with wickedness.

Gen 6:5  Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually.
Gen 6:6  And the LORD was sorry that He had made man on the earth, and He was grieved in His heart.
Gen 6:7  So the LORD said, “I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them.”
Gen 6:8  But Noah found grace in the eyes of the LORD.

In contrast to the wickedness of those around him, Noah found grace in the eyes of the LORD because he was righteous and walked with God.

It is of interest to note that the phrase “this is the genealogy of” occurs in over half a dozen other places in Genesis (and elsewhere in the Bible).  Following this phrase, the sons of the individual are listed.  For example:

Gen 11:10  This is the genealogy of Shem: Shem was one hundred years old, and begot Arphaxad two years after the flood.
Gen 11:11  After he begot Arphaxad, Shem lived five hundred years, and begot sons and daughters.
…

Gen 11:27  This is the genealogy of Terah: Terah begot Abram, Nahor, and Haran. Haran begot Lot.

The same thing is done with Genesis 6:9.

Gen 6:9  This is the genealogy of Noah. Noah was a just man, perfect in his generations. Noah walked with God.
Gen 6:10  And Noah begot three sons: Shem, Ham, and Japheth.

The last part of Genesis 6:9, “Noah was a just man, perfect in his generations. Noah walked with God.”, is a very brief explanation of who Noah was.

Conclusion

Noah was perfect in his generations means that he was righteous in contrast to the wicked around him.  It has nothing to do with his bloodline.