No; in states which allowed slavery, the law was always that a child born to a slave mother was enslaved, regardless of facial features or skin hue, unless and until the master freed him/her.
The censuses of 1850 and 1860 required enumerators to subdivide African Americans (slave or free) into “black” or “mulatto”. This is obviously imprecise and we can’t be sure how much attention any enumerator gave to this classification.
Nevertheless, for what it is worth, the census of 1850 classified 411,000 (10.4%) out of 3.95 million slaves as “mulatto”, and 177,000 (36.2%) out of 488,000 free blacks as mulatto.
Assuming the enumerators took any care at all in their work, we may safely conclude that the 89.6% of slaves classed as “black” didn’t look like Britney Spears and couldn’t pass for white. Given the choice between “mulatto” and “black”, an enumerator classed them as black.
Of the 411,000 enslaved “mulattoes”, we have no way of knowing how they looked. No doubt they covered a wide spectrum from “slightly lighter-skinned black” to “looks like white but I’m not sure”.
Note, however, the larger percentage of free mulattoes. There was a tendency for lighter-skinned people to become free, for a variety of reasons: masters were more likely to manumit children who “looked like them” (mostly white), white-looking slaves found it easier to run away and pass, and masters didn’t always want white-looking slaves because they wanted dark-skinned slaves to believe that race and slavery were synonymous.
Given which, if I had to guess, I doubt that more than 10% of enslaved “mulattoes” (1% of all slaves) could have passed for white, even on a good day in the right light. Maybe 20% and 2% at the outside.
Of course 1% of 4 million is still 40,000 people.