That is not a good sign, but the M47 makes some debris, and some of it is not magnetic, so some of what you were probably seeing was from synchro debris in the fluid as well. It does get very dirty, although I've never seen any manual transmission fluid get black like you described.

Run for a few thousand miles with the fluid you bought and put in, and then do what oz sez to do... put in Redline MTL if you can get your hands on it. If not, you can use synthetic 10w30 motor oil too. If you have that awesome quantity of metallic debris in the fluid next time you drain it out, then you can start shopping. But why not see what happens first.

If you're doing a lot of city driving, expect to see some brass or bronze debris in the fluid, which is from the synchros. When I drive all highway miles, the fluid stays metal free. When I drive in the city a lot, it loads up with synchro material after a while. But I get very little metallic debris on the plugs over 20-30k miles.

I change my M47 fluid about once a year with Redline MTL. That's 50,000 miles on the fluid. Except last time I ran out of Redline (thought I had more than I actually had) and I topped it off with a quart of Type F fluid, so I'll probably do it again the right way next weekend after I can get to the place that sells Redline oils.

Also, when you fill the transmission up, tip the car on it's right wheels so you get more fluid into the transmission case. That keeps everything lubed better than just filling to the level of the hole while the car is on level ground, so I'm told.