Okay let me see if I can answer this. Black inkjet ink is normally done with some kind of pigment which could be charcoal. But that’s all the color it will have. Whatever color hues the pigment has the ink has.
Now when we work with mixing inks it is normally called subtractive color theory. Black is actually created by mixing in all the colors together until you get black. Of course we know it will only really get dark brown in practice. Therefore a black pigmented ink is needed.
With printing CMYK it work a little differently. This could be called additive color theory. Again in theory if we print CMY together we get black. But it is neither really dark enough nor rich enough. Therefore we add K which is the black printer. Sounds like the same thing and it is in relation to the black not being able to be achieved by mixing colors.
On the contrasting side the black pigment has no others. When we add CMY to the K then the black turns a richer darker black. Additive theory is all about what color is absorbed by the printed pigments. A color rich black absorbs more light.
I don’t know if that made any sense.... LOL. Here is an article on color theory and mixing inks with added links at the bottom of the page:
http://www.catspitproductionsllc.com/sc ... ixing.html
But in the end a color rich black is far blacker than just printing the back alone.
And I have no idea how you print your transfers twice... LOL. I could never get one to go through the printer the same twice ;(
I hope that helped.