What are some of the reasons to doubt that our minds are "blank slates"?

There are some.

Explanation:

Some use to say that, but the real fact is that it is not true, according to some recently made researches our minds are never ever "blank spaces" just because it never stop working, even when we are on sleep mode the mind works, the use of energy and it's potencial can be a bit lower but never stop. Some say they just, really, stop when life is out ou our bodies, as it is an automatic machine tat must keep functions on all the time, if it's not like that our bodies die.

Common sense, cultural universals, and scientific studies of twin are compelling reasons to doubt the theory.