So The Heugar live underground the Blaine live above ground... The Heugar are addicted to ichor, but the Blaine get the same buffs if they choose to use it? Hmm.. sorry a little odd but ok.
99.99% of the Huerghar you see do not live underground and haven't lived undergound for generations ever since at least the conflux (pre-MO1 timeline). The Blain seem to be just a separate evolutionary line of the Oghmir that had to adapt to Nordveld when they became stuck there (as others theorize, due to the Sundering separating the Huerghar that were in Nordveld from the ones in Myrland). We don't know much about this but I suspect that although the Blainn do have some immunity to Ichor within them that they would not go as far as the underground Huerghar who are rumored to bathe in Ichor.
I think this question however arises from a "lore questionable" clade tree. You are basically asking: "why can the Blain use Ichor pipes, lore suggests that maybe it should have been restricted to Huerghar" and maybe you are right and maybe you are wrong - I can see it both ways in this case.
What I can't see is why Blain don't have a clade that gives them cold resistance or why a Kallard can have both cold and heat resistance, simultaneously to make things worse. It has been suggested many times that although it is fine that a clade shares many gifts at a high level that each specific "sub-clade" should have its own specific gifts but unfortunately as far as I am aware no one at SV ever commented if this is something they don't really care about or if it is something they intend to do eventually but just not a priority right now.
Sheevra live below ground mostly yet have dark skin, and Veela live above ground and are fair skinned... again a bit confusing but ok.
It has been a while since character creation time but I do seem to remember you can create a pale Sheevra and a dark skinned Veela