You could do something like this:

1) Get all blogs that has attribute=true
2) Get all tags for these blogs

Code:

            BooleanCriterion booleanCriterion = new BooleanCriterion();
            booleanCriterion.Value = true;

            BlogQuery blogQuery = new BlogQuery();
            blogQuery["Attribute"] = booleanCriterion;

            BlogCollection blogCollection =
                QueryHandler.GetQueryResult(blogQuery);

            //Get tags for the first blog
            TagCollection tagCollection = blogCollection[0].Tags;

Karoline