I believe that is a new trend and not one that is really being embraced by everyone.
People recommend not using them because they look too similar to teal pumpkins in the dark. Teal pumpkins is for kids with allergies or other food restrictions. The teal pumpkins are for the child's safety, while blue ones are not.
The blue color being associated with autism also has roots with Autism Speaks, a very controversial organization.
If a child with autism is struggling with trick-or-treating, then their caregiver or sibling should be there to help them. That is how it was when I was a kid. And I have had trick-or-treaters come to my door who were disabled and their caregivers were helping them. We don't need the blue pumpkins.