And seriously, think about what it really mans to take offense at the phrase "it's okay to be white". What would you think of someone who gets offended when people say "it's okay to be gay"? What would you think of someone that gets offended when people say "it's okay to be an immigrant?" Why, then, do you think it is at all productive to take offense at the phrase "it's okay to be White"? Whites will be just as offended as gays or immigrants, and rightly so.
Also, as far as I know the Spanish never called themselves "blanco". The Spanish word for lighter skinned people is "rubio" (though this is sometimes also used to refer to blonde hair). White Spaniards - using the modern understanding of the term White - during the conquest of the New World called themselvs "peninsulares" (after the Iberian peninsula) or "corillos" [1].
As far as the unscientific nature of the term "White" I don't find that particularly relevant. Race is a social construct, not a scientific determination. It would be just as (un)fair to criticize Black Lives Matter for referring to an unscientific concept.
Furthermore, the concept of "the white race" is unscientific, and was created to group together a majority against minorities. Italians, Jews, even the Irish were consider "non-white", while the Spanish conquistadors began calling themselves blanco in order to state their superiority over the aztecs.