There's no point in dealing with generalities here. Digglez is right in that there is nothing inherent to blunt force that makes it better for breaking shields. However, taking a more weapon-specific view of the issue does show a few cases where mace-like weapons are better than swords against shields. The warhammer, for example, is heavy enough that, especially when using its spike, it would do more damage to a shield than a sword (
http://www.youtube.com/watch?v=qhknaG9ifbs - a 2h warhammer being used against plate armour). As already mentioned, a morning star also packs a lot of weight behind a couple focused points which could puncture and fracture many shields.
But this is all secondary to whether or not it's a good mechanic for balance. I'd like to see more than just the morningstar and axes given a bonus against shields. Perhaps a system of 5 stars to indicate the effectiveness of specific weapons against shields. Most axes would be five stars, for maximum effectiveness, while morningstars and spiked maces would be 3-4, weighted weapons used from horses would be 2-3 (for a wrecking ball effect), and basic swords would be 1.