Were your mother's pearls black or very dark when she bought them? I know these were bought as light blue/silver. I do not believe the color has changed much since she bought them.
I remember my mom took her pearls out in 1994, to show them to me since that is when I started in pearl culturing. She had this strand she said was black but looked just like yours.
Hard to say...you know how memory works: oftentimes it just doesn't! Perhaps they had always been that same color, but she remembered them darker. At least that is what she told me. She gave the pearls away to friends when she divorced: they had been presents from my dad, so she did not want these, so she doesn't have the pearls anymore. She only owns Cortez pearls now. Go figure!
Wish she had taken a photo: before/after. Cameras -especially ones with good macro lenses- were not as pervasive as today!
All I can say those pearls had been irradiated, because -just like yours- some of the pearls had cracked and peeled around the drill hole and the bead was darkened with a blueish tint to it.
I had a customer -decades ago!- and she had one really black Akoya pearl necklace, a few pearls had cracked. I could see the blackened bead, but the nacre was also darkened. I found a photo (really lousy one, since my camera back then was a Sony Mavica with 640 pixel resolution (and a floppy disk for storage!) of one of these pearls. Both the nacre and the bead were very black.
I can imagine these pearls were double treated: irradiated (hence the bead is black) and dyed, since the nacre coating was also black. I always wondered about these pearls.