I'm not sure I understand - if you are saying Sagar is the sire of those "very close to black" foals, then they are smoky black (not sooty black) - black with a single cream. And yes, they are very, very often mistaken for black, as it's not common for them to look anything otherwise. Some do (look up Nightlight by Guaranteed Gold) but most don't.
And yes, that can make cream hide for generations if foals end up being smoky black as well.
There have even been some buckskins who are so dark they never are suspected to be as much - look very much like regular brown or bay.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET