Unfortunately while Premium E-Liquids are unique and full of flavor for the most part you may notice they are also a bit sweeter. The extra sweetener and additives tend to burn a coil out quicker as these ingredients are what speed up the carmalization process that happens on your coil. The black gunk you see is just caramelized sweetener and flavoring not really anything to gross other than the way it looks at times depending on how often you change the coil.
We typically see around 2 weeks average on House Blends and anywhere from 2 days to a week on Premium E-Liquids but not all are terrible on coils, some can provide a good week and a half or even 2 weeks but it is a hit and miss. Usually desserts, maple syrup, candies are the worst when it comes to this. Fruits tend to be okay more often than not.