Different from ripening? Ripening is a process in which enzymes break down cell walls, acids, and tannins, and break starches into sugars, isn't it? Bletting appears to be a second one.

For whatever reason, ripening is defined as a hormonal process generally triggered by ethylene. Bletting is different from that, and happens after the hormonal ripening. It's more of a biological distinction than a culinary one, I think.