There is currently no way to search commons materials only by tag, or title. Searching for more than one tag increases your pool by adding more results containing either tag, instead of only the results containing both tags, or in many cases, similar words.
The more specific a search term, the less likely results are to be actually related. Search for the music theory term "tonal harmony" for example yielded few music related results, but included materials related to subjects including biology and George Orwell. Search for the astronomy term "redshift" turned up no astronomy results, but included materials ranging from paramedic training to a picture of a biscuit.