The question's a bit of a mismatch, honestly. Mangadex is a manga hosting site, not a reading app, and as far as I'm aware, it doesn't have an official app with offline download functionality. The term "blame" might be getting mixed up with the app 'Tachiyomi' and its forks, which could use Mangadex as a source extension. That's probably the path people are actually talking about.
For true offline reading on Android, TachiyomiJ2K or its successor Mihon were the go-tos, letting you pull manga from sources like Mangadex and download chapters ahead of time. Since they're gone from the Play Store, you have to sideload the APK, which is a whole process itself. On iOS, the options are even slimmer; you're basically looking at web readers that maybe save to your browser cache. It's a bit of a fragmented ecosystem, not a single app solution.
If you're deep into scanlation reading and need offline access, you end up cobbling together a workflow. Maybe downloading chapters manually from a site and using a generic comic reader app. It's not elegant, but it gets the job done when you're on a plane or somewhere with spotty internet. The official platforms for the series you want usually have their own apps, but that's a different, often paid, world.