Even though it's about time travel, they have still tried to keep things realistic in regards to attitudes, actions and so forth.
In 2007, Freema Agyeman joined the cast as Martha Jones. Within that series, there were two episodes set in the past and they both had the characters from that time make reference; In "The Shakespeare Code", Martha herself referenced it, as did Shakespeare. Then, in Human Nature/Family of Blood, it's referenced again with people making fun of Martha for it, and talking down to her because of it.
These are alright once in a while, but if it were to happen in every episode, either they ignore it which is massively disrespectful, or it becomes an issue in every episode which would be tiresome and would detract from the point of the show.
It's really a tricky proposition.
Personally, I'm not against it if they can find the right balance, and they hired the actor for his merits alone, not based on skin colour.
Also, they have referenced the clothing warn in historical episodes on multiple occasions.