Never heard about this before, and seems it's maybe related to "Flashblood" where Heroin users would pass on their blood right after injecting, mainly to avoid withdrawal? From 2010: https://pmc.ncbi.nlm.nih.gov/articles/PMC4407801/

> During 2005, a new blood-sharing practice, “flashblood,” emerged among female IDUs in Dar es Salaam. Flashblood is a syringe full of blood drawn back immediately after initial injection that is passed to a companion to inject. Those practicing flashblood believe that the syringe full of about 4cm3 of such blood contains enough heroin to avoid the pains of withdrawal.