Black diamond is the best activated carbon I have ever found and is very safe for your tank. I don't think anyone lies in this hobby, they just get allot of misinformation. There is allot to know, seemingly. Once the knowledge is required, who wants to go back and learn it all again (except for crazies like me). Your retailer is probably only getting paid minimum wage and is probably more conserned with who they look than diving into endless research for a second time, or the first.
Activated carbon removes organic molecules, copper, trace elements, and heavy metals. With out activated carbons these would would grow to levels that are unsafe. If you use ozone in your skimmer than it is crucial that you use carbon after the skimmer so ozone doesn't kill you tank. Carbon is misunderstood only by those who haven't used it.
Black diamond is the best, before adding it to your filtration put into a filter bag or wrap it in filter floss. Rinse the in the sink until the rinsing water is no longer black and is rinsing clear. You can hear black diamond hissing for accross the room. The hissing is a sign of high quality carbon. It is best effective after the skimmer in my opionion and experience.
Good keeping...
