Blocking workflow using Twinkle:
https://drive.google.com/file/d/1F7EmD9yjpXJ6yK3pT8H2-LDiv08orUbf/view?usp=sharing

Some general notes:
Twinkle presents an interface that lets the user select one of numerous (very numerous) reasons for blocking, as well as the duration of the block and several other options. It also simultaneously posts a predefined template onto the offending user's Talk page.
The value that Twinkle adds is doing all of this in a modal window on top of the page being read, never taking the administrator away from that page. It very likely uses API calls that we can use ourselves, if we want to build a similar feature, although many things about Twinkle (such as the collection of predefined Talk templates) is probably highly specific to English wikipedia.

If we want to send the user into any kind of blocking workflow inside a WebView, it can only be the standard Special:Block page, which only does the blocking bit. (we can't encapsulate the literal Twinkle workflow)
Leaving a note on the target user's Talk page would need to be a separate step, to be done sequentially afterwards.

Because the policies for blocking vary wiki-to-wiki, we will wait and revisit once we have been able to work on T358265: Set up Community configuration for Edit Patrol or if we hear further requests to add blocking (have only heard 1 so far).