Are blending “plants” a legit thing?

I’ve only had to go there enroute to a refinery drop if my gravity was off too much with the transmix/black diesel trash they load on me. Last load was at 30 and had to pump it back because they couldn’t get the load up to a sellable spec.
yeah I’m learning so much about stuff I’m prob assumed to fain ignorance to it’s not even funny.
Do other legit crude carriers like Plains, etc…have to do this or is the stuff yall come outta the field with up to a spec that doesn’t have to get blended?
Dude that trained me that left for a better job yesterday told me they had to be careful where they sent some of these loads lest they get blackballed which I why I assumed they called off the 30 weight load.
They other reason why I question these blending plants or maybe this one is because of the amount of paper in/paper out write ups that happen there with stuff that never even gets offloaded but goes straight to the refinery UNLESS they have to load something on top of the load to get it to spec.
Maybe this is a normal thing I don’t know but it literally reminds me of how drug dealers add anything to a base drug to get it to weigh more without reducing the noticeable potency but increasing their volume/cost in the process without destroying their rep.