In the welcome message for this site I state that colonial definitions of indigenousness result in all kinds of deprivations and plagues on our peoples. Blood quantum as a measure of indigeneity is such a definition. Its intent is clear in the texts of the time when the idea was invented.
A good example is this quote from Historian Patricia Nelson Limerick in the book "The Legacy of Conquest: The Unbroken Past of the American West":
"Set the blood quantum at one-quarter, hold to it as a rigid definition of Indians, let intermarriage proceed as it had for centuries, and eventually Indians will be defined out of existence. When that happens, the federal government will be freed of its persistent 'Indian problem.'"
Check out the discussion section for a debate on this topic between colonial and anti-colonial thinkers. The discussion is called, "Indigenous Knowledge Theft".