My understanding is that ATProto itself is definitely decentralized but the app view most people interact with using the Bluesky app is centralized ...sort of. The Bluesky app view will read from PDSes hosted by other people, hence people on Bluesky can see stuff posted elsewhere, like users of Blacksky. If the Bluesky app view decides to stop reading from any other PDS (like those of Blacksky, or ones which are self-hosted) they're free to do so. The same is true for alternative app views like Blacksky. Since most people think of Bluesky as the thing you see on the official Bluesky app (which shows the Bluesky app view) an outage of the Bluesky app view will mean they lose the ability to view any posts from any source. If someone's using a separate app view like Blacksky, the most that will happen to them should be that they'll lose interaction with posts coming from Bluesky's PDSes until the outage ends.

I may have the division between Bluesky and Blacksky off, but ATProto does allow this sort of thing. Hosting a PDS is trivial and requires very few resources. Hosting a full app view can be expensive depending on how many PDSes you're ingesting from, but you can decide how much of that you want to do.