Blacksmithing is a little more versatile than JC for being able to get the gems you want regardless of available slots, however I think you will get a lot more out of JC on the whole; you can actually make money with JC because everyone needs gems but with BS your market is smaller and so are your profit margins... is also highly convenient being able to cut your own gems, which is the same reason why Im also a big fan of Enchanting.
Enchanting is a really rewarding profession BTW; you never end up in a dungeon group without an enchanter, you can DE your BOP gear for profit when it gets replaced, it is its own gathering profession unlike BS or JC, you can save heaps of gold by doing your own enchants. Yes, the only ring enchant for tank is stam which is unfortunate but honestly its not as bad as it could be... I used to be a BS / miner now BS / ench; I only dumped mining because the stam bonus was worthless for my DPS spec.
BS / JC are the best combination of professions for tank because they both give you a permanent +80 mastery, whereas you can only get mastery from Alchemy by using elixirs which can be quite expensive relative to flasks. I totally understand if you have other reasons for wanting to keep alch, but if you are looking to optimise you are just as well off dumping that as mining.