Thank you for your suggestions.
You are right that you cannot filter the Archive widget to reflect categories/tags for blog posts. There is currently a feature request in for this which you can vote on here
In the meantime the only viable option is a custom widget. Sel Arom discusses how to implement such a widget in a blog post here
We don't have a scheduled date for when this feature will be included but it is on our list.
I would also suggest you to review this discussion
in our Sitefinity forum.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items