What steps should be taken if one believes they were injected with black market juvederm plus (silicone)?

Contact Allergan and discuss your concerns with them. If you received a product that was not Juvederm the clinic will most likely be investigated. Best, 

To investigate the authenticity of injectable filler, you can contact Allergan (the company that makes Juvederm), and have them investigate your case. They should have a record of your lot number and the purchaser. The FDA can also investigate these cases, but I think Allergan is a good start.