What does Blackmoor have that Mystara doesn't?

[Havard]

If I wanted to learn more, are there any old classic D&D products for Blackmoor? Or do they all start with AD&D?

Yes. More importantly, there are no AD&D products for Blackmoor. The BECMI line had four modules for the setting, called the DA series:

DA1 Adventures in Blackmoor
DA2 Temple of the Frog
DA3 City of the Gods
DA4 Duchy of Ten

The above modules are made for the BECMI/Mentzer edition, but are usable with any version of Classic D&D and easy to adapt to AD&D.

In addition there is the First Fantasy Campaign, published by Judges Guild for Original D&D, also a very valuable resource for the setting.

Although requiring more adaptation, it might be easier to get your hands on the 3E or 4E Campaign Books for Dave Arneson's Blackmoor. These also provide a really good overview of the setting and can be used for the older editions, although there are chapters in those books that will be more or less useless if unless you play that specific edition. Still, they might be worth your money.

-Havard