FAIL - this one was a bit iffy for me, I really felt torn both ways, but I went with fail because although to me it was an obvious color reference, I wouldn't have thought to look at the parents based on this line alone and I'd consider what the father has passed on as well. Especially since white genes are not as strong so just having a white parent doesn't imply majority white children.
FAIL - I love love love your non-traditional unicorn reference, and once explained it made perfect sense, but there wasn't enough of that to guide me to that answer prior to reveal because I was still thinking color not other traits/species.
Or maybe you are? Blood runs thicker than water. - History ref: He is still a pure unicorn, with pure unicorn parents.
PASS - The saying itself definitely made me think of lineage/history which was spot on here.
Magic ref: He also has magic that involves blood.
PASS - Definitely the blood once more helps point to Sacre and his blood magic, but blood alone wouldn't have made me consider this, however since his blood magic has to do with dehydration I associate it with water which again works with this particular phrasing.
Appearance ref: He has a blood marking on his flank.
PASS - Love the color ref and with the saying making me think of lineage kinda goes hand in hand with color coming from family genetics and stuff.
Do you ever wonder what would become of you? - Fluff.
Fluff.
Your name is Robert. - Name ref: When googled, Sacre results in multiple Robert Sacre entries.
PASS - Now that I'm awake, typing in SACRE does yield Robert Sacre, herp derp. Easy peasy.
Overall a Pass. Great lines!