Do Blasts Pass Through Obsidian?
I LIVED.
Based on this, it appears that yes, obsidian 'blocks' the effects of explosions. My health meter flashed, and my character said Oof! a bunch of times, but no damage was taken.
Anecdotally, from the explosion patterns on another question, I never saw a block that was further from the center being destroyed while another, closer block wasn't. All my target blocks (redstone) had 0 resistance, so anything touching them should have popped them (though they do have a sort of resistance when they get destroyed).
The mechanics as described on the Explosion page over on the Minecraft Wiki state that they are modeled as rays originating from the explosion center travelling out until they are fully attenuated. If they all meet an impenetrable wall (obsidian), they can all be absorbed and nothing gets through.
Depending on how you define "passing through", because explosion has different effects on blocks and entities.
According to the Minecraft wiki article on explosion, entities (including you the player) will always get at least 1 point (half-heart) damage if they are within blast radius (TNT: 8, creepers: 6, fireballs: 2), regardless of explosion exposure (calculated with sophisticate algorithm, or simply how much you're covered by obsidian). And the damage can be absorbed by armor subsequently.
The above statement is reproducible for me with a 1.5_01 client. However, the main part of that article was written by me based on disassembled code, so I can't provide more verifiable sources than this.