I came across this plant at a local nursery and it was labelled as Philodendron. Without any species or variety information.
Now I am almost sure that it is not a "Black Cardinal". I think it is either an "Imperial Red" or a "Rojo Congo" but at this point I cannot decide. Even after checking countless images of both.
Here are some pictures of the leaves, stem and the whole plant. For reference the pot was a 21 cm one, so the plant was quite large.