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If you've made the mistake of selecting Blacksmithing as your primary profession, then you will have no choice (no reasonable choice, anyways) to select mining as you second primary profession.
If you do not, you'll never have enough WoW gold to spend on anything but buying metal bars at the Auction House to level up your blacksmithing skill.
Now, mining is an easy skill to level. Just mine whatever node you come across during your adventures and your skill will almost always be sufficient.
Blacksmithing is another matter entirely. Mandorall, who is a level 70 Paladin, has a blacksmithing skill of 268 or thereabouts, and he got that high – which is not very high at all, no precious – by the skin of his teeth.
The amount of materials needed to level up the blacksmithing skill, especially above 200, is mind-boggling. You will be using mats worth 20g to make a single item worth maybe 5g at the auction house.
The problem, at the point where I'm at, is that the only things that will level my skill are "Blue" items which require enormous resources to produce. Now, the items produced are not bad, but since everyone has has blacksmithing HAS to make those items to level up their skill, the Auction House is always glutted with them. Here is the low price of these items. You know, supply-and-demand, basic Economics 101.
I'll be the first to admit that I regret my blacksmithing decision so much that I have not looked around to buy recipes that would maybe make leveling up my blacksmithing skill a little easier on the wallet. However, I have characters with Leatherworking and Tailoring and even at the levels where they are – around 330 – there is always a recipe that will produce a level point without requiring a second mortgage on the Epic Mount. That, as far as I know, is not the case with Blacksmithing.
I would recommend to anyone considering Blacksmithing as a profession to think again. Is your wallet going to support that extravagant profession? Is your patience for farming up to the task? Or are you going to end up a penniless husk of a toon, begging for coppers on the mean streets of Stormwind?