Here's a device that plugs into the Blazer's 12VDC receptacle and gives you a digital read out on the real time voltage. It also has a red alert if the voltage is 'LOW'. It also has two USB ports for charging your smart phone and/ or tablet. The dual ports are: 1./ 12 watt/2.4 volts and 2./ 5 watt/3.4 amps.
I bought it on the net from Frys.com. If you want to see this product ,go to Bravo View Technology for model no. CGR-VM34
I paid 19.99 for it and about 3.00 shipping.
My only problem with it is the two recpt.s on my 2003 Blazer 2 door are low on the ash tray and are not in the line of sight while driving. IOW I wish it was higher on the dash to be easily visible.

I used the Digital Voltmeter when charging the battery on my 96 Lumina 4 door. It showed me when it was charged fully, i.e. 12.6 VDC. Not sure if this is a 'correct procedure'. I cut all electrical devices off when charging a battery. But with this D.V. in it didn't seem to hurt either the charging of the battery or the Digital Voltage plug in. OP.

 

I have used this Dig. Voltmeter & dual charger on my Chevy Lumina as well. With the engine off or when it is idling, it reads a low 12.0 to 12.3 VDC. Then when you get the engine above idle and the alternator comes on line, the volts jump to 14.0. That shows me the alternator is good and doing it's job. The battery is getting weak and is on the way out. I have 'slow charged' it twice now and both times the low voltage described above returns. All that info for $23.00!

 

I still have the original plug-in 12 V receptacle charger and it is still displaying accurate DCV info and still charging my smart android. I like it. I do not leave it plugged in when I shut off the engine because it will run down the battery. The 12 V receptacle is always on so I use it only when the engine is running. Oh, yes, you can easily switch it from car to car to truck.