First off I have to say that I love BlackInk
I've noticed that while working with an adjusted opacity, using the eye dropper tool to select a different color resets the opacity back to 100%. I'm unsure if this is intended, but I suggest that a LOCK opacity option be added so that while working on a large/multi-colored piece you can lock the opacity at a given percentage while switching between colors and using the eye dropper tool.
I was working on a rather large piece that required a adjusted opacity for blending and while switching colors, using the eye dropper, every time I'd have to go back and readjust the opacity. It added time to my work and ended up becoming too tedious; I dreaded adjusting the opacity every other stroke, so I ended up putting the piece on hold.
I'm assuming, that with a custom brush I could have gotten the same result with full opacity, so that I didn't have to go in and reset it, but I'm not familiar enough with the custom brush side of BlackInk yet.
A simple lock opacity option or button would be a helpful addition; or you know, stop the eyedropper from resetting the opacity all together.
That's all!