Hi Jim and welcome to the forum. The official MB term is Active Blind Spot Assist. IIRC this feature was included in one of the Driver Assistance packages you could tick as an option when specifying either the Premium or Premium Plus trim level. Of course this was in 2016 when I was speccing my 220d Sport. It is designed to work in conjuction with Active Lane Keeping Assist. The link below explains in detail how they work:
Blind Spot Assist explained
Blind Spot Assist explained