According to halacha, the earliest time you are allowed to recite the tzitzit bracha is when there is enough light outside to distinguish between your white tzitzit strings and your techelet tzitzit strings. Since today we do not have techelet (although there is room to disagree with that statement), I had always assumed this halacha didn’t apply.
But now I just came across a post by a Chabad rabbi by the name of Rabbi Moshe Miller who asserts you can use the black stripe on your tallit instead. In other words, when you step outside, if you can make out the black stripe on the white backdrop of the tallit, you would be allowed to recite the bracha.
In fact, he suggests that is one of the reasons why the custom to have black stripes on the tallit developed.
But it seems to me that you would be able to discern the black bands several minutes before you could make out the difference between the thin white and techelet tzitzit.