Here's an interesting theory, Professor Birch discovered the Dark type and was the first Professor to have knowledge about the Steel type.

With the confirmation of chronological in-game order(RB=RS > GS=DP > BW > B2W2 = XY) it can be assumed that Professor Birch was studying the dark type when being attacked by a Poochyena at the start of the Ruby and Sapphire games and since Red/Blue and Ruby/Sapphire happened at the same time, Professor Oak had not yet found out about the Dark type.

As for the steel type, Steven Stone had been researching and training Steel type Pokemon (Becoming the master of the type in the process and the champion) for Birch's research on the newly discovered type.

Birch then sent this data to the other Professors and hence 2 years later in Gold/Silver and Diamond/Pearl the types are known.

Now of course this is taking into account that Remakes are Canon (FR/LG had Dark(Moves) and Steel types.) and this is the only way OR/AS will have Mega-Evolutions and have the Fairy type. (Unless they make it an entirely new game of course)