It can be done. Below are the steps that you should follow.
1. Create a new class that inherits from TagsList
2. Create a property where you will get the blog IDs so you can create a filter
3. Override InitializeControls and do not call the base.
4. Here you need to create a custom data source and bind TagsRepeaterList control. The custom data source should contain the filtered data from the specified posts. You need to loop through all posts from the specified blog and check the tags using GetTags method.
IList allcontent = tag.TaggedContent;
// your logic here
You may find the following post useful - filter categories by post which describes similar approach.
the Telerik team