Everyone else will be confused about what the hell this "Heroku" thing is.
That said, if they don't use appropriate error codes (maybe 502 or 504 for Heroku issues and 503 for app issues?) they should. But I don't think error messages should mention "Heroku" by name.
Delusional. If it's down, it's done, and unless your product is a developer tool, >99% of people won't care why.
For those kinds of customers, they may understand what Heroku is and why their vendor is using it and will definitely make at least some distinction about outage fault.
By Heroku not listing when its their downtime, they are insulating their reputation as a hosting company from end users, at the expense of the customers already using them. It's a little shady.
I agree that the average end user would probably not care, but most not caring does not mean it's not valuable information to some people. So I see where the original poster is coming from.