Remilia Scarlet, also known as the Scarlet Devil, is a vampire living in the Scarlet Devil Mansion. As would be expected of a vampire, she drinks blood. The crimson color of the tea is reminiscent of fresh blood.
It is unclear what country the Scarlet family is from, but since they are a line of vampires, there are many who believe that they may come from Eastern Europe.
The Scarlet last name and their employment of maids make one think of English nobility. The black tea they drink only adds to that. While you are free to think whatever you like, digging too deep into the Scarlet family's roots is a bad idea. You might end up with a silver knife cutting through time to pierce through your body.
If you end up lost in the Scarlet Devil Mansion, the first thing you should do is greet the mistress, Remilia. You can be polite or you can be rude, but if you have a smart head on your shoulders, you should already understand which is more likely to prolong your life...
If Remilia takes a liking to you, she might tell you what teas she likes. She might even say, "We have a long night ahead of us, so let's have some fun." If this happens, you should join her, unless you have an urgent reason not to.