I picked up this tip while watching Purge teach Day9 (a great series).
For low-tier players, a Blink Dagger isn't really the best item. Why? You frequently get it put on 3s cooldown due to bad positioning, you tend to Blink too far or into deathtrap positions, and it's hard to save up for, because you have to buy the whole 2250 at once.
So... why not a Shadow Blade? It's uncommon for lower-tier players to remember to invest regularly in Dust, and thus a ShadowBlade acts as a much better escape, especially if you've already been initiated on. As long as you survive, and they forgot the Dust, you can get away clean. Furthermore, it aids your attack speed and damage, unlike Blink Dagger.
The ONLY time Shadowblade is worse is if you need to gap-close or re-position instantly, or if you want to initiate into tower range. While you should not underestimate these facets of Blink Dagger, especially as your positioning-knowledge gets better, I think Purge had a good point.
After trying it myself, I was almost tempted to get BOTH. I really liked the relative safety and ganking potential of Shadow Blade, but I also missed the insta-position of Blink Dagger.
Side note: I should also mention the Force Staff. While you may sneer at it because it's a lower-range Blink Dagger, it helps many initiators get the mana pool they need, and it gets you, or your allies, out of tight spots. It's an easier build-up than Blink, and about the same cost. I recommend buying the Staff of Wizardry component first, and if you change your mind, there are lots of other things you can build into.