In the situation you're in, where the point is to use a distinguishing feature to describe the person and you can reasonably trust that they have some African heritage, the best term is "black". This is, by no means, unanimous. You will find some people who feel that any distinguishing of a person by their appearance is inappropriate. Sometimes, however, it becomes at least useful to be able to do this. You may also be better able to use the term as one of several -
The person I saw was a beautiful, tall, black woman with long, straightened hair.
Note, it's one thing if you're talking about them to someone else. It's another to use this term for them when you're around them. The best person to ask this question - is your coworker. If you inform yourself, you'll be better able to share your information and avoid offending them.
For some levity in a generally serious situation, I'm going to call on our friends at Saturday Night Live who, just this week, briefly addressed this in a special summer "Weekend Update" in a commentary to the NAACP. At the very beginning of this segment, Michael Che, a black comedian, speaks:
The National Association for the Advancement of Colored People has issued its first ever travel advisory. Warning people of color of discrimination in Missouri. First of all, NAACP, stop calling us "colored people". We picked a color a long time ago - it's "black", okay?
While I don't think that Michael Che speaks for the entirety of people of African descent, he's got a loud voice.
As I said before, some people think that these words are inappropriate and that colors how people act because they don't want to sound racist. I'm going to guess that this contingent is why your coworker acted the way they did. When you read things like this article on The Spectator, it makes it difficult to know how to describe someone who is black. I'll note that this is from the point of view of a white British male.
Apparently the correct term in the USA is not even ‘black’, but ‘people of colour’. Forgive me, but there is no meaningful semantic difference between ‘people of colour’ and ‘coloured people’. Why does one offend, then, when the other does not? Isn’t this a case of confected outrage and manufactured sensitivity?
The other problem with "people of color" is that it means "anyone not white", which isn't useful to describe a black person in your situation. If your coworker said "person of color", they could be talking about someone who is Asian, Hispanic, Indian... anything non-white.
I found an interesting discussion with a story nearly identical to yours, though asking a different question:
I understand why White people do this–fear of being labeled as a Racist. My question is, does it really offend Black people if someone describes them as Black? Or is that just White people’s perception?
One responder, Emmaline, makes this related remark:
I use the term "Black" because not everyone identifies as African --even if their primary ancestors may have originated from Africa, their family has lived for many generations in the Caribbean Islands, for example, and only identify with that culture.
Someone else, sativaah responds to this
People of Color (POC)
... There are a lot of opinions in that thread, so you have a lot to consider from a variety of people.