Many players find that they get the best results by combining different types of pickup, taking advantage of the particular strengths of each. For instance, many players like to use an internal microphone to capture the resonance and sonic depth of the guitar’s body, with a piezo system used to capture the detail and attack. Or, if the piezo sound is too up-front for that guitarists’ particular musical goals, a magnetic pickup and a microphone will give you a very full, round, ‘big,’ natural sound with good detail.
Most players don’t blend piezo and magnetic pickups, because both methods are driven by the strings themselves, so you risk the possibility of clashing frequencies and all sorts of sonic weirdness. So in short, if you want to blend, you'll be using a microphone pickup blended with something else - the question you need to ask is 'why?'
If your style is more rhythmic, a piezo & mic combination will give you a great representation of your guitar’s attack. If it’s more detail-oriented, a magnetic & mic combination will be more representative of what happens to the string throughout the whole life of the note.
There are some piezo pickups (such as the K&K Sound Pure Mini) which are placed on the underside of the guitar top, giving it a much smoother voice than a regular under-saddle piezo. Seymour Duncan’s Mag Mic combines a hum-cancelling magnetic pickup with an omni-directional condenser mic in one unit.