Hey Andy,
Appreciate you posting your question here.
If the Blogmaster is using a free theme AND adhering to the
GNU/GPL License then he or she would sustain reference to the theme source which typically appears in the footer or at the very least in the source code commented out. Sometimes the Blogmaster removes this reference which is a breach of the license.
If the Blogmaster is using a purchased theme then typically reference to this theme is not required so it would be at the Blogmaster's discretion as to whether or not reveal that information.