Do I need a better Alternator???
 

So as of now I have a full sound system in the blazer. 2 amp, head unit, eq, subs, and speakers. I also have LED turn signals and I have a HID kit on the way. But right now I've noticed that the headlights dim and get brighter as the LED signals blink when the turn signal is on. Is this a signal that I need an Alternator with a higher output?

what kind of subs and amps are you running? if your pushing over 1500rms i would suggest you install another battery in the rear. or you can buy another alternator just make sure you get a good brand no cheap ebay ones... with your led turn signals are you using a resistor with them? if not you will be not giving the full power to the led and it will burn out quicker, now to the hids ive never seen power loss from any hids i have 6 hid 50 watt kits and my voltage with all 6 just goes down a little bit not enough to notice.

How would I wire a resistor for the LED's?

Depends on the power, Just do the big 3 upgrade (info on that can be found with a search) and it should fix your problem, I am running 750w at 2 ohms right now on my system, custom built LED tail lights, (90 per tail) along with 65w halogen headlights with a heavy duty harness, which pulls directly from the battery, and I do not have a single problem with my headlights dimming.
So I think if you did the big 3, you would be good

BTW, it would be best to use voltage regulators for the LED's instead of resistors. Keep it at a constant 12 volts, and throw a couple diodes on the input and output on the voltage regulator to give a nice flat power output to the LEDs. they will last a VERY long time then, even longer than resistors.

*edit*
Did you get plug and play bulbs (the bulbs that fit directly into the sockets)
if so, they already have built in resistors and you do not need to worry about resistors or v-regs.
the only time you need to worry about v-regs and resistors is if you have built a custom set.

For the most part the LED turn signals were plug n play. The only thing I had to do was figure out which wire color matched with the old turn signals

Do you have a link to the LED's that you used for this? Even though they are probably fine the way they are.
I think with the big 3, you wont see anymore dimming of your lights.
You never mentioned what kind of power you are running to your sub/s (RMS)

The subs get power from the amp which has a power lead connected directly to the battery and here are the turn signals 98 04 S10 Blazer Sonoma Pickup LED Bumper Corner Signal Lights Smoke 00 01 02 03 | eBay

Well, obviously! LOL
how many watts RMS do you have going to the subs?

Probably about 650W for each sub considering the amp is 1700W