It seems a bit strange to me that in 1.10 the obsidian upgrade is the lowest tier. In previous version I believe obsidian was just below diamond. Was this intentional and if so, why? It seems like a bug.

It was intentional, because the usual case in modded Minecraft is obsidian is trivially obtainable and renewable. Iron and gold didn't have much use outside of the first ... half hour of play?

But the low-tier obsidian upgrade adds bonus blast resistance in addition to the 2x multiplier.

That's fair. It is pretty easy to get obsidian in a tinker smeltery. :)

I also didn't know about the blast resistance bonus.

Thanks for your time!