Is rust inside a black iron natural gas pipe normal?
It's entirely normal.  There is always a bit of moisture in natural gas and there can also be small amounts of other impurities including oxygen.  This is enough to cause small amounts of rust in black iron pipes.
Obviously there is little danger of it rusting through any time soon since, as you've noted, it's been there perhaps 60-70 years and will likely be good for another 60 years or longer.