I am not familiar with a blackening process. I just painted them with Testor's gloss brown. Thanks for the white glue on a thread idea! I'll try that the next time
Blackening is a simple process. I used it on the brass gun barrels on the spar deck of my Constitiution.
I put WHITE vinegar in a saucer, lay the photo etch pane in and let it sit for a few minutes to remove any release oils on the brass. It is acidic so a few minutes is all you need.
Clean the vinegar off with clear water and put the panes on a paper towel to dry out. Wrap the pane so both sides dry.
Using Blacken It in the same manner, a little in a saucer, letting the PE soak...but it helps here to use a tooth pick to move the PE a little in the solution. In about 4 minutes your PE will be a dark bluish/black. How dark it becomes is determined by how long you soak it. You can reuse the Black It so don't heave it overboard.
You are now ready to glue or paint your PE.
With Tom's PE for the Titanic, I soaked the sheets one by one before separating the individual pieces, since they are so small. But when I cut the individual pieces off, I had bright brass spots where they attached to the sprue. I touched those up with some semi gloss black Testor's I had laying around.
I let them set for 48 hours before gluing them to the hull.
What ever you do, do not use the good china saucers as this tends to promote some severe conflicts. Women do not seem to understand the modeler's world!