As far as I know Bluesky protocol is open-source but not its main app. Also, there are several apps available to use Mastodon. Additionally, note that Bluesky uses the AT Protocol (developed by Bluesky) while Mastodon uses ActivityPub (W3C standard) which is more friendly for third-party devs.
As far as federation is concerned it’s still mostly centralized with Bluesky while Mastodon is fully federated.
Something I personaly like as well with Mastodon is that the feeds don’t use algorithms.
Not being a developer myself, the wording may not be the best sorry. The take home message was that Mastodon seems more prone to external development based on its protocol and the fact that more than 20 apps already exist (all platforms included).
Outside of using several apps to use Mastodon, I personaly rely on its API only to add Mastodon posts as comments on my static blog.