Are you talking about just the blackening or making the 45 deg bino's.
As far as the later goes the July August edition of Sky and Space has an article called DIY giant binoculars along the same lines.
This guy used 2 x 100 mm f5 Acromatic scopes and 4 x 11/4" stardiagonals and 2 newtonian diagonals but if you read this you will get the general drift on how to do it.
I already have 3 of the cheap star diagonals as you tend to get one with ever scope you buy these days.
I like the idea of using mirrors as i think there is some colour being induced from the prisms in these bino's as using the Radian with out the prisms yelded very good images.
You do lose light on ever reflective surface tho unless you use very expensive dialectric mirrors.
With the price of the ED80's coming down , they would make a nice higher power pair although some what longer
.
You can even get 120mm f5 Acromatics for $500each????
Good for lower powers with more light gathering.
Man I love making stuff
As far as blackening the insides it's a bit involved as far as removing the lenses and prisms and would not like to give a how too incase you stuff them up.
Then I would be the worse @!$
in the world.
I found them pretty straight forward but am used to pulling my stuff apart.
The main thing to remember is because of the fine plastic threads you have to be carefull not to cross tread them especially on the front lens guards.
Price $249 including freight from Andrews.
For that price there good value but not up to Japenese standards or even that of there larger units which appear to be a little better constructed.
These have definaltly been designed with weight in mind.