Create a View of Group Posts Who's Group has a Specific Tax Term?
In addition to your view configuration, you need to set up a group node relationship and filter out group nodes by taxonomy term. This should do it for you:

Add relationship: Organic groups: Group node (post)
Require this relationship: checked

Add filter: Taxonomy: Term
Vocabulary: Your Vocabulary
Selection type: Dropdown
Relationship: Group node (post)
Operator: Is one of
Select terms from vocabulary Your Vocabulary: blog