My first thought when I see blue eyes in a solid black is self chinchilla. That's what you get when a chinchilla allele <
c(chd)> in the dominant position, which suppresses yellow pigment to produce a chinchilla agouti (agouti
A_) or silver marten (tan
a(t)_), combines with self alleles <
aa>. Two self alleles prevent expression of any markings at all, so the chinchilla effect is undetectable - except for in the eyes. Sometimes they'll get blue-gray eyes; sometimes they'll have brown eyes; and sometimes the eyes will start out blue and then turn brown as they mature. That means when you're only breeding selfs - like SF are - that chin allele can hide for a
very long time. This is experience speaking - I had self chins for four generations before I figured out what was going on with my occasional blue-eyed blacks!
Blue eyes in a black rabbit can also come from the vienna <
v> which makes blue-eyed white (BEW), or dutch <
du> alleles, but in those cases you often see other markings on the body, as well. However, the vienna can express as one or both blue or marbled eyes with no other white markings, or with scattered white hairs - which of course you would not notice on a SF!
So, my first guess is that there was a chinchilla somewhere in the past. It could also have been a BEW, but I think chinchilla is probably more likely since there are a few large meat breeds that are bred in that variety, which could have been crossed into the SF lines; but there is only one that is bred in BEW (the Beveren, which is pretty rare).
As far as I have observed, simply having a dilute allele, which when it is doubled <
dd> makes a blue or lilac with blue or pale eyes, will not produce blue eyes in a black when there is only a single copy of it.
