If it's not completely black this sounds like bad color developer, or incomplete/inproper re-exposure. I would guess they botched the process somehow. It couldn't be your fault if the leader is red. I would bleach fix it. If it works, it was their fault with the bleach exhausted. If it doesn't, it was their fault because the botched the development/re-exposure somehow. It's harder to diagnose with slide films as it could be a combination of about 6 factors, but my money is on bad developer. If the images were dark red and the film was black, that sounds more like a bleach issue.
Find a lab that runs film a lot. If they run a roll every other day it's a crap shoot. I do just do it myself for the 95% cost savings I get. The E6 process specifications/process includes a chemical fogging step, but when I'm developing sheet film I will frequently skip that and just expose it to the light. It doesn't matter, but with roll film you have to take it off the reel, which is a pain, or at least make sure that you get all the film which is difficult with anti-halation layers. They most definitely used the chemical version, but that could have been exhausted.