Cutting and pasting but preserving context:

>>>> low intensity dark blue

>>> Phew!  I wouldn't have to come up with high intensity dark blue.

>> Hmm, dark blue refers to quite far into the edge of UV, i.e.
>> 450nm-ish and lower. Now I understand the joke in your posting.

> I've always understood "dark" or "light" to refer to intensity.  The
> "blue" part refers to the hue.

Your understanding is understood.
As a joke the comment is somewhat obscure and therefore somewhat 
excellent :-).

But, using the term "dark" and "light" to differentiate colours of 
differing but
similar wavelength or mix of wavelengths is also extremely common and 
time honoured.

Googling:

"dark blue" "light blue"            - 694,000

eg

"... Dark blue is associated with Oxford University and light blue
with Cambridge University."
____________

"dark green" "light green"        - 282,000
_________

You'll find in the vast majority of references that degree of
saturation / brightness is not the point of differentiation.

FWIW Many colours can not be produced from a single wavelength but are
a mix of pure wavelengths which our eye-brain systems assign a
"colour". The relative brightness of the 2 or more combined pure
wavelengths are varied to form various "colours", but for a fixed 
ratio of intensities the "darkness" or "lightness" of the
resultant colour is not dependant on the combined absolute intensity.

See eg      http://others.servebeer.com/temp/ColourChart.jpg

    CIE Chromaticity diagram circa 1931!
    (Taken from Luxeon LED "all in one" app note.)

Note that using any 3 Luxeon LEDS does not allow a good rendition of
the whole of the colour space. The triangle formed on the CIE diagram 
using RGB LEDs leaves out much of greeny-blue area. Using any 3 other 
single wavelength colours also leaves out substantial area. By adding 
CYAN to the usual RGB mix you get an irregular quadrillateral which 
includes much more of the CIE colour space & so (obviously) greatly 
increases available colour range. Using as "dark" a blue as possible 
improves the range of possible colours produced. If only 3 LEDs are to 
be used to produce as wide as possible a range of colours then a 518 
nanometer emitter plus as "dark" a red and as "dark" a blue as 
possible would be ideal. This is not available from Luxeon - the 
nearest are cyan at 505 nm and green at 530 nm. The perceived colour 
difference between these is large.



        RM


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