Jim - a huge proportion of my pen output is oak, and both Shaver and Bill Mooney have it right.
As Bill said, we spend our time trying to get super smooth finishes, but as Shaver said, oak is an open grained timber. The two facts do not fit well together. However, it can be achieved, bit do not blame your tools, it is probably not their fault, its just a question of adjusting your working practice.
If you want a super smooth finish, firstly do your initial sanding with the lathe running, using coarsest grade you need to, which is probably going to be 120 until you get a bit more experience. Stop the lathe and using the same grade, sand with the grain until you have removed all of the radial marks left from the rotational sanding. Apply diluted sanding sealer (about 2 parts sealer to one part thinners is a good open mix) wipe it off and allow to dry for a few seconds. Sand with next grade - lathe running. Apply sanding sealer. Sand with next grade - with the grain, lathe stopped. Apply sanding sealer. And so it goes on. I find it only necessary to sand oak to 600 grit, but do your last sanding with the grain but without any sealer applied.
You should now have a super smooth finish, without any discernible remaining traces of sealer, ready to take your chosen finish. BUT, and this is the difference between success and failure with oak, the very last thing you should do before applying your finish, is with the lathe stopped, give the blank a vigorous rub along the grain with a tack cloth, which will remove the black marks you mentioned which stick in the grain. Hey presto.
For myself, I take the attitude that smoothness of oak is directly proportional to effort involved, which is directly proportional to price of pen. Therefore, my slimlines don't generally get such elongated treatment, but my high priced fountain pens do. I quite like the open grain effect on some pens anyway, and as it turns out, so do my customers. Each to his own eh? But that's how to do it if you want a smooth surface and still retain flexibility for the final finish.
I hope that makes sense.
