Let's start with the one constant benefit of blacksmithing that lasted through the entire expansion. Like its close cousin jewelcrafting, blacksmithing is probably the most flexible craft in terms of profession bonuses. You get two more colorless gem sockets than anyone else. How worthwhile is that, though, really?
In terms of raw itemization points, it's not necessarily a huge advantage. Most tradeskill character bonuses tend to boil down to an additional 80 attack power, 47 spellpower or maybe 60 stamina. These two colorless gem sockets are consistent with that level of crafting bonus. What makes blacksmithing's bonus extra special, though, is that you obtain the crafting bonus by using gems, which means any stat that's available from gems is available to you for your crafting bonus at about the same itemization cost. If raw attack power isn't your thing, you can focus on strength, agility or even haste. It might not sound like a big deal, but it's a nice little side bonus on top of everything else.
So, the crafting bonus for blacksmithing was pretty nice. What was very important at the beginning of Wrath of the Lich King?
At the opening of the expansion, back when we expected Naxxramas to be bleeding-edge content that would keep raiders well-fed and challenged for a while, blacksmithing's goods were in high demand. There was a time the stats from tempered titansteel felt godlike to a tank, and I can't even tell you how long I carried the Titansteel Shield Wall. I carried that bloody thing into Naxxramas like Ernie carrying his rubber duckie into the bath. "Titansteel shield, you're the one. You make raid time lots of fun." I'm pretty sure I was sporting the Spiked Titansteel Helm as a DPSer until some time in Ulduar. Don't even get me started on the Titantsteel Destroyer -- that bloody mace was a major weapon back in the day, if you weren't an active raider.
All this gear was incredibly useful when Wrath of the Lich King started. If you were a fledgling raider, every bit of strength, attack power, stamina and spellpower felt like it could mean night and day when you were facing down new bosses. That meant that the extreme cost of smelting titansteel bars, finding a blacksmith who had somehow managed to get his hands on enough material to level to 450, and then fund the cost of the weapon ... it was all worth it. Heck, it was darn near mandatory when most raids were at the beginning of the raid curve.
Unfortunately, this blacksmithing nirvana didn't last for long. While the gear was certainly helpful to people who were just getting started with their raids, Blizzard had provided us a new tier of raiding content that was more accessible than any other raid in the history of the World of Warcraft. And while the blacksmithing gear was awesome, it wasn't so much better than Naxxramas gear that it would be relevant to folks who could reasonably expect to clear Kel'Thuzad. And even your average PUG could reasonably expect to do that.
At the opening of Wrath, then, blacksmithing had two roles. First, it would be briefly helpful to bleeding-edge raiders who were making the first stabs at Naxxramas. It would also be useful to those groups who were struggling with learning how to raid and wanted every advantage they could get. But once players could expect to zip through Naxx, they weren't going to need our anvil-related skills.
And then Ulduar and ToC came along. Now, both had blacksmithing patterns, and some of those patterns would prove to be best in slot to certain characters. However, that gear wasn't so easy to create that blacksmiths could expect to simply disperse them out to the populace for profit. That meant our big money-makers turned out to be the Eternal Belt Buckles and whatever odd titansteel item you could sell to an alt. (I usually made more cash by providing the raw titansteel bar, though, instead of the finished item.)
Ulduar and ToC provided much better gear than your average blacksmithing pattern, and the raid-gained patterns were expensive to anyone who wasn't part of a successful raiding group. This was our period of what I call "kinda useful." Blacksmithing was awesome if you could scrape together the recipe and materials to make the Breastplate of the White Knight, for example. But if you couldn't, then you were just a belt buckle dispenser.
Finally, though, at the end of Wrath of the Lich King, as the screaming Horde and hollering Alliance beat their way into Icecrown Citadel, there was a renaissance for blacksmiths. The Icecrown craftables hit the market for the cheap-at-twice-the-price cost of a primordial saronite. You could obtain this blood-of-gods by either hitting up your local 25-man raid or completing your daily random dungeon.
This meant that any blacksmith who wanted to put in the effort to obtain primordial saronite (including picking it up from the local auction house) could start to create contemporary armor again. While most of the Icecrown blacksmithing patterns would be obsolete to many 25-man raiders, that crafted gear would be the highest item level gear available to casual or 10-man raiders. In other words, blacksmithing had hit a pretty good relevancy again.
I feel like this last period was the golden time for blacksmiths in Wrath. The gear that we were making was important and useful, but it wasn't the best in slot gear for most folks. It was situationally appropriate -- good and advantageous, but without rendering (most of) what you were doing redundant.
This final level of balance is what I'd hope to see in Cataclysm, that happy land of "good and useful, but not best in slot."