I have an iPhone 7 that's blacklisted. I didn't steal it, although my guess is that it was abandoned by someone who did steal it. It does not work on any US network.
From the 13th-23rd, I did a trip to Central Europe. I used this SIM from Three UK in my day-to-day phone:
I took the blacklisted phone with me in case I lost my day-to-day phone, so that in an emergency I could still do VOIP calling via TextNow or Talkatone and receive iMessages via wifi. And yes, I was planning to see what would happen if I plugged in a European SIM into the blacklisted phone.
On my last night in Europe, in Warsaw, I decided to swap the Three UK SIM into the blacklisted phone. It worked fully! I was able to get on all 4 carriers that apparently have roaming agreements with Three UK. I kept the SIM in the blacklisted phone and used it in Munich before taking my trans-Atlantic connection. The phone worked in Munich too.
So now i'm back in the states. The SIM is in the blacklisted phone, and it still works for data only in the U.S. via roaming on T-Mobile and AT&T (not Verizon) - voice and SMS doesn't work in the U.S. with the Three UK plan that I bought.
This morning I checked the phone's IMEI with this site that clearly confirms that the phone is blacklisted:
3 (Three) United Kingdom | Carrier Check - IMEI.info
So, I'm now really curious as to how stringent the blacklists truly are. I would have thought that a US carrier blacklist would easily apply to the EU, but apparently not.