It's pretty clear that no state would be willing to use black market drugs for executions.
fentanyl isn't black market. it's a Schedule II painkiller ("high risk of abuse, recognized medical purpose").
and besides, the research chemical markets are more 'grey market' than they are black market. the Federal Analog Act rests on shaky legal ground, and is rarely used to prosecute anybody.
instead, the DEA goes after specific designer drugs by placing them in Sched I on an "emergency" basis, which is essentially permanent. the process is reactive: a highly popular designer drug will be out on the clearnet for 2-3 years before the DEA bans it. see
mephedrone for a good example. the synthetic cannabinoids (K2, spice, AB-FUBINACA) have met a similar fate.
the State of Texas could probably buy grey market fentanyl derivatives/imitations without breaking any law, or at least without being prosecuted.