When is a black person racist?

In a philosophy class in school, I was taught that a racist is a person who renders negative moral judgement against another person based on the color of their skin. Since skin color is not a matter of choice, this was regarded as an irrational basis for moral judgement and thus, well, immoral.
In the service, as a trainee, I was taught that skin color was not relevent, since we were all equally stupid, unmotivated, soft, lazy and worthless.
In combat, nothing matters as long as somebody follows orders, can shoot sort of straight and keep their shit together.
I liked thinking that I did not give a shit about race.
But I have noticed that when I bitch about the current president, some people accuse me, a white guy, of being racist.
Especially on Fluther.
How, exactly, does that make me a racist?
Is a black skinned person who bitches about a white president also a racist? (It seems that it does not).
I have noticed that when, on occasion, an attempt is made to hang the damaging label “racist” on a person with black skin, most blacks and some whites sort of act like the premise is ridiculous.
At what point can a person with black skin be a racist?
If there is a double standard, when does it kick in?
If I dislike the president, but love my non white girlfriend, am I a racist, or a hypocrite?
What label do we hang on black people who like white presidents? Will they always be stuck with being called porch monkeys or Uncle Toms?
Are those labels racist?
Am I talking about racism, or is it something else?