Asphalt bleeding is a phenomenon in which lots of little bits of asphalt start to move up to and through the surface of the paved area. If the bits of asphalt break through the sealcoat and accumulate on the surface, you may see a shiny black line that looks almost like someone spilled and tracked oil over your driveway.
This bleeding is not only ugly, but it can be unsafe and also indicate that your driveway badly needs work. If not fixed, the slick line could allow someone to slip; the breached sealcoat could allow moisture in, which could further destroy the asphalt surface; and the asphalt could actually start to break apart, depending on the reason why the bleeding is happening. All in all, this is something that you need to address immediately, and the first step is to determine why it's happening.
When Did This Start?
Did your area just have a massive heat wave, one with temperatures that were way above normal? Did the bleeding start shortly after the beginning of the heat wave? Then the heat is a likely culprit. Asphalt is made in varying strengths, for lack of a better word, that allow it to withstand a certain amount of environmental assault. If conditions become too extreme, though, such as in a heat wave, then the asphalt can react badly, and the bleeding begins.
If there was no bad weather and the bleeding happened gradually, then it's possible the binder in the asphalt was faulty. Asphalt is made up of lots of aggregate material -- all those little pebbles -- that is bound together and sealed. If the wrong binder was used or if the binder was applied incorrectly, the surface isn't going to hold.
In most cases, repaving the surface is best. Once you have that slick material on top and a potentially disintegrating layer below, it's hard to fix the asphalt. Repaving is easier. For heat-related cases, you can see how long the bleeding goes on and whether more heat waves are forecast. If you decide to have the driveway redone anyway, ask about getting asphalt that can handle higher heat.
Obviously, this is not a DIY job, so contact asphalt paving companies and get quotes for repaving your driveway. Don't wait on this because bleeding asphalt is not something you want to trifle with. It should be fixed as soon as possible.