I usually don't post that often but ..Whenever Melissandre refers to royal blood, it seems to only refers to lineage tied to King's Landing throne. So Ramsey would not fit.
But it's interesting. I mean, Robert Baratheon became king through a rebellion, not by birth right. How would that confer him and his bloodline, "magic properties"? Did he descend from the Targaryens (or at least Stannis + Gendry)? Or was it due to something more enigmatic like "destiny"? One of the many things not explained yet.
Edit: I actually expanded my question while you put your comment. Interesting. Yeah, that might be an important spoiler.