Im completely new to pedals, but i wanted to do what seems like a simple mod to swap the LEDS in my Boss Pedals, and put in Blue ones.  How can I do this and where can I buy leds that will fit and look good?

You will need to get the same size LEDs. Check out Mouser.com for replacement blue LEDs.

Good Luck!

I thought blue LEDs had a forward voltage of 5v (as opposed to Green/Red having 2v). Would this mean changing the series resistors ?

Torchy has a good point.

Each color of LED has a different "forward voltage". This is the minimum voltage across the LED required for it to light up. Red LEDs are typically around 1.3 volts, greens are usually around 2.3 volts, and blues are higher still at 3.4 volts or so. These are all rough numbers because every chemistry of LED is a little different and there are a couple dozen different chemistries out there.

Every pedal has a resistor in series with the LED. This is called a balast or series resistor and its job is to impede the flow of power so that it gets limited to a maximum value. Some pedals go a step farther and add a zener diode in series with this resistor to stop the LED from lighting as the battery gets low enough to affect the operation of the circuit. Depending on the circuit that turns the LED on, you might have to pull out the zener and make the resistor larger to compensate.

As you change colors of LED, you'll probably want to adjust the resistor to keep things in balance. Newer LEDs are more efficient and therefore require less power to make the same amount of light. As long as you prevent more than abour 20mA from flowing, the resistor you choose is totally up to you. You may have to adjust up or down, but the stock resistor will be a good starting place.

I hope that babbling explains something. :?

Oh... also keep in mind that LEDs are polarized so they'll only light up in one direction. If you don't get light, swap the leads.

Best of luck on your mods!

Take care,
-Peter

http://www.theledlight.com/ledcircuits.html

usually blue ones are brighter than the crappy red ones that come in boss pedals and actually need less current for similar brightnesses..... in that respect id just swap the leds and forget about the effect of the difference in forward voltage as the intensity difference will most likely make it a desirable brightness anyway.

On this same subject,  I replaced the small red led in my Boss GE-7 with a 500mcd/1.85V red 5mm one.  
I'd like it to be brighter than it is.  Should I get a higher mcd rating or should I change a resistor in the pedal to do this?

both would work...you can just make the resistor smaller to allow more current, keep in mind it has an effect on battery life too... a brighter led (given the forward voltage is the same) would use the same amount of current but give a brighter result