A modifier, not a base coat by itself.
The coat must be a dark shade, meaning it falls somewhere on the bottom half of the chart(s), or the middle region for Nighted.
Bloodstained and Skinned can appear in Nighted and Albino, however Bloodstained does not affect Nighted, and Skinned does not affect Albino. Bloodstained on Albino and Skinned on Nighted will make it a grey.
A marking or two may be outside the range of bloodstained as long as they don't dominate the design.