There would need only be one mutation, say Bob is the first to be born with blue eyes ( he probably would have caused quite a stir, wouldn't he), he then has children and passes his blue eyed gene onto his children, but his mate passes on the brown eye gene, since one theory says that only one correct gene is needed to create brown eyes, the children would be brown eyed, but would carry a blue eyed gene as well.
One set of genes from the Father (blue) and one set from the Mother (brown).
Their children have children, and as it is random chance to which gene they pass on, brown or blue, in later generations the chances increase that a child will again end up with two blue eyed genes and have Blue eyes. Quicker in a smaller population base, due to intermarriage. Then the whole cycle starts again.
If both parents are Blue eyed , theoretically they should not be able to give birth to a brown eyed child, they would not have a working copy of the brown eye gene, yet they do have brown eyed children. So it is thought there is another gene involved that has not been found yet (green eyes?) that also plays a part.
This Common ancestor (or the Blue Eyed Adam/Eve), and Most Recent Common Male/Female Blue Eyed Ancestor (the latest individual we all share in our family tree with blue eyes), would be two different things, as is Chromosomal Y Adam(the earliest common male ancestor), and MtDNA Eve.
Genetic testing will probably find other common ancestors who introduced genetic mutations into the dna, some of which are all inclusive and others which cover only a subset of the population, can you say lactose intolerant.
You are related to a lot more people than you think, the first 20 generations ( about 400 years at 20 years to a generation, though the US avg is now about 25 years) above you are something like this 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576..(odd how it follows the original terragen image ratio, isn't it)... so some of my 1,048,576 ancestors who comprise my 20th generation MAY to be in your line. Go further back, say 3000 years and they HAVE to be in your line (as the increase is exponential 80 generations, 1600 yrs, is like a trillion trillion people, if none of them were common, so they have to be), These are not your traditional paternal or maternal surname ancestors most people track but all parents of parents of parents,,,
SeerBlue