Color changes on succulents are not uncommon, and the same is true for Aeoniums. However, it looks like this is an Aeonium ‘Emerald Ice’ and the coloring isn’t usually that dark on the edges.
You mentioned that it used to have more leaves but that they turned dark and fell off. Normally darkening would indicate rot, but the way the leaves are laying downward, rather than firm and toward the light, makes me think it isn’t getting enough water.
That said, this could also happen if there was a sudden change in sunlight, such as being placed outside for a while. Aeoniums like it cool and brightly shaded. If they get too much sun or heat they will dry up quickly and get sunburn, which looks similar to the little spots I’m seeing on the bottom of the leaves here.
It is natural for the lower leaves of a succulent to fall off as it grows, but if they’re falling off rapidly, it’s likely a watering problem. Aeoniums tend to grow more in the fall and spring, and will sometimes shed a lot of leaves before they begin actively growing again.