I don't have any info on the % chance of each chest appearing, but I did compile a list of all 3 mystery chests last fall. I haven't checked to see if any changes have been made to the contents or their percentages, but I don't think so:
Blue Chest(Rare) | Gold Chest(Epic) | Purple Chest(Legendary) |
1xMana 10% | 1xScroll 15% | 1xTradingStn 10% |
1xGum 10% | 1xGate 15% | 1xGenie 10% |
1x15%PP 15% | 1xArmorer 20% | 1xLibrary 20% |
3xCC 25% | 1x50%PP 20% | 1x50%PP 30% |
3x5hr 40% | 2x8hr 30% | 3xTeleport 30% |
The Wiki defines them as Rare, Epic, and Legendary. Since that same language is used for Blue, Gold and Purple bands in Instants (plus a green band for 'Common') I would guess: Blue has a higher percentage chance than Gold and Gold has a higher percentage chance than Purple. What those actual numbers are, I have no idea.