You were given incorrect information. They appear to be hatchery grade Black Australorps, but definitely are not BSLs. I've raised BSLs for years and as you can see in the photos below, BSLs have reddish or copper colored feathers around the neck.I got two like this at a sell yesterday they have no red on them as I can see and no red on the face but I was told at the sell they were bsl pullets
Googled from http://chickengardener.com/wp-content/uploads/2010/10/black_sex_link-e1286339073512.jpg and https://www.dunlaphatchery.net/birds/Standards/img/Black-Sex-Link-Chicken.jpg.