I had washed and conditioned it once, but it wasn't helping with the light spots. Since the ink spot on the front was kind of bothering me anyway, once I decided to dye the black, I figured it would take more evenly on unconditioned leather-and I could also attempt to get the ink out at the same time.
It's worth mentioning that the light spots weren't that noticeable, and I could have lived with them, it was as much to experiment with the dye as anything. I wanted to start with a clean slate, no conditioners.
The Fiebings is super thin, pretty much like black water. As I said, they dye just soaked in. After conditioning, I could still see the lines between the touch ups and the undyed, but only if I tried really hard.
So here are the afters:
The piping around the corners, this was the worst part of the bag:
The back of the bag, for some reason the glare in the picture makes the bag look lighter than it is, but it shows the contrast a little better, you can kind of see the black dye, but just barely.
Here is the full bag, it's not a very good reveal picture, it looked better on my phone, and it looks as black in real life as the above pictures where I show the piping:
Overall I'm really happy with it. You can really see all the flaws out in the sun where I took the pictures, if you're looking for it. Indoors it really looks like a new bag. I'm going to try buffing it again tomorrow afternoon, I still get a little black on the cloth when I rub it down. I'm going to wear it with black pants the first time I use it just to play it safe, but I suspect that it's not really going to be a problem. Will report here if it is in case anyone else is tempted to try the same method.
Amazing improvement, congratulations!