IIRC, horns on the Blazer are inside the LF fender where they're fairly easy to reach.

This is an opportunity for you to learn how to use one of the basic tools of electrical diagnosis. You could use either a test light or a voltmeter, but the voltmeter is a much more powerful tool and if you do any amount of electrical work, you will need it.

Test light:
Simply unplug the lead to one of the horns. Clip the lead from the test light to the metal clip that connects to the horn. Ground the spike at the end of the light. Have a helper hold down the horn button. If the light bulb inside the test light illuminates, there is voltage going to the horn, so the problem is not the circuit, but a bad horn. If no light, no voltage, so chances are the problem is before the horn.

Voltmeter (an inexpensive Radio Shack digital model is most handy):
Get voltmeter - read manual. Go back and read instructions: this time, actually READ the manual. OK.
Unplug the horn lead - set the voltmeter to read DC voltage (AC voltage is house current). Insert the red (+) probe into the horn lead, and ground the black (-) lead on the fender in an area which is bare metal. Have your helper hold down the horn button. If you see either a reading of 12 to 14 volts, or a reading in the same range with a minus sign in front of it (just indicates the leads are connected backwards - no problem) then you are getting voltage to the horns, so they are faulty. If no voltage, then just as before, the problem is before the horns.

Voltmeters are darn handy. Test lights are very limited but also handy. Get both.