Threatened to be blacklisted because i refuse to pay for something i did not agree too

So the service from Skin renewal illovo was very good, no complaints, but after some surgery, they asked me if they can send my tissue for testing at lancet laboratories, at the time I assumed it was for research, they said they just needed my permission so I agreed( no mention of costs were mentioned or on the paper i signed). 2 weeks later I get a bill of 750 rand that i owe to lancet, this is after i have not received anything from lancet, i have no idea what they even did or if they even did anything, i have received nothing from this company but I am now being threatened to be blacklisted because of my refusal to pay for something i did not even order or agree to