irradiance of blue LED light
First specify WHY you want to measure this.
Second define exactly what you want to measure along with resolution and accuracy for each wavelength.

e.g.  Is it relative to the Eye or to spectral energy. 
    These are not the same thing.


If you wish to measure the Eye response level then use an Ambient Light sensor which is a PD with Eye spectrum Matching optical filter.

http://www.digikey.com/product-detail/en/panasonic-electronic-components/AMS302/255-2655-ND/2125641

Then consider the ambient light that needs to blocked like a camera with a lens, recessed aperture, focal length and calibrate with a precision distance understanding the sensitivity to positional errors.