I vote sharpie. Make a dot. Although it should be done from the inside. The plastic will decrease the blackness if done on the outside. I am certain that no sharpie will fit into that tube. It's super skinny. Much less reagent than the TFP tube. The mixing bottle is 14ml total.
Seems to me that only real way to fudge it would be to sharpie a dot onto a piece of paper and stuff it into the tube. Or maybe some clear contact paper.
The block needs to be replaced. Not cool. I'm not sure anyone has ever reported this issue yet. You win first prize!