Blood is always red. Except when it isn't. Human blood, anyway. This is true even though blood fairly quickly curdles and scabs to a brownish color when exposed to air, to the point that chocolate syrup is often recommended to fake scabs. Even literary description of the stains, if they are not shown, must specifically say that the stains are of blood in order to be understood. This is probably done as a bit of visual shorthand, to let the audience (and other characters) know that the stains in question are actually blood, as opposed to other things that stain brown, such as mud, chocolate or feces.