If your OS is set as DE does Blocs not run as DE? Or you just want only Blocs as DE?
You can still use Mojave that’s not an issue and hopefully that will remain through the entire life cycle of Blocs 4.X.x. Though if you want to change the language it’s just not as easy per app (as outlined in the Blocs help docs) currently in Mojave, that’s all. Otherwise you’re fine using Mojave in general.
In regards to Blocs general logic on OS support that can be seen briefly here.