My pure mage is only level 11 and he's currently traversing the Dwemer ruins that will inevitably lead him to Blackreach (where he is to obtain a quest item). I've been playing on Apprentice difficulty ('Normal', I suppose) because this is really the first time I've tried a pure mage build in all of my decades of RPG playing (on paper and in video games) and I'm going to try DiD. I also have Sven as a follower, but he has been proven weak against the more formidable opponents so I can't really trust him to save my butt if I get into trouble. The Kinect has made it a hell of a lot more fun and efficient, but still I'm always worried I'm going to get one hit. My last build (a pure fighter) traversed Blackreach on Master difficulty at level 40 or so and there were some challenging episodes, so I'm wondering if my young mage may be biting off more than he can chew. Should I turn back (I'd say I'm about halfway to the actual entrance of Blackreach now)?

Probably. There are Dwarven Steam Centurians that will probably own your mage. Also Apprentice is Easy, Adept is Normal

make sure you bring plenty of potions and a better follower with you, the dwarven centurions are the biggest headache when you're a low level, you could bypass blackreach altogether if you want, but will miss the crimson nirnroot quest, just take your time and pick off the falmer with a bow also works !

bring a pickaxe, theres geodes everywhere that can be mined for soul gems.

dont worry about the centurions, i think theres only 3 in blackreach and can be avoided altogether by not getting close to them and activating them (they'll stay dormant if you keep your distance)

If you have the full unrelenting force shout, get close to that hanging glowing orb and fus.ro.dah it

Now that this is in Cheats, Hints and Spoilers, I can be a little more specific.
Dwemer constructs are a pain for a Mage, but they can be avoided by a little trickery. They do not climb or jump, and will try to get to your character by the most direct path.
The entrance to Blackreach can be opened without having to actually defeat the Steam Centurion in Blackreach, and there are other people in the Alftand Cathedral who can be used as Meat shields, but they will also try to kill your character.
The Falmer Boss midway through the Animonoculary will be the toughest opponent for any character.
Alftand is not a good Dungeon for any mortal followers, because it has a lot of places for the follower to get lost or fall to thier death.
Bring a lot of lockpicks, the chests may contain some enchantments that will be impossible to find when your character has a higher level.

Potions and a beast of a Follower (someone who wields a two-hander would be my suggestion...) and you could be ok.

Mind you that Falmer are inordinately tough regardless of your level. They seem to be set at a higher level in relation to your character level than any other enemy.

Go low and slow, try to take them on one at a time, and be prepared to drink potions like a Nord drinks mead and you should be ok.

Only one Centurion gave my low lvl character much of a problem. But it activated as soon as I hit the top step, perhaps a good enough sneak would have gotten her past it, I don't know. Did finally manage it, w/o a follower BTW by thowing a spell..run...throw a spell ..run.... til it was... out of steam. As long as it can't "one shot kill you" running around up and down the steps or finding another really good spot might work. My character was about lvl 15 so not that much higher.... BTW that Centurion was the one before you get to Blackreach, I have never activated any of the ones IN Blackreach.

The Falmer on the other hand... Ouch! ... did make it through though.

Yeh it could be a bit rough at that level so take lots of health potions. I ended up there at level 1 when I chose one of the options in a mod Live another Life, it placed me in a room in blackreach and it took me several tries to get to one of the lifts to get out.

Ah... Well honk my hooter. I'm going to bump it up to Adept then to dirty my pants even more.

Thanks everyone. I'll decide tonight what I'm going to do. The responses do make it clear that there are some tough spots and since I'm playing, or trying to, DiD, I want to make sure I'm careful. The quest item is not a totally significant quest, just one that I was doing to get in good with someone at the Mage's college and receive a reward. So, I may turn back and try this one again after doing some other quests.

Leave now. You will be killed by the falmer.

I must have missed in your earlier post that you're doing a DiD run. In that case, you may want to have a couple of other characters in back up ready to go... lol

But if you can survive (I recommend liberal use of the Fury and Frenzy spells in the Illusion school...), then like someone else mentioned, there is a veritable fortune in geodes to be mined for Soul Gems.

Ah just use Sven as bait and kill the centerions ans they are killing him.

Bummer, I never noticed you could mine soul gems.