And you can also say that something such as a knife is "dull" meaning it is not sharp.

They are synonyms (same meaning), but "dull" is not as common as "boring" when speaking :)

Or along the same lines as Julia's answer something can have a dull finish if it is not shiny or polished.

You can use dull in these colloactions too: dull sound (not clear or sharp), dull color (not bright), dull sky (cloudy and gray)

The dictionary is a powerful tool. Did you look these two words up?