I love to use a slanted brush, and I do not necessarily use black eye liner. I use different colors, especially green, purple and blue. I love L’Oreal HIP Color Rich Cream Eyeliners, especially Teal, found in any local drugstore, Walmart too!  
Another product that I recommend is Urban Decay's Eyeshadow Transforming Potion $18. The wand features two different brushes that connect to two separate vials of Eyeshadow Transforming Potion. The potion then transforms any eyeshadow to a liquid liner. The double-ended magic wand allows you to create either a thick or thin line.
If you need a visual, check out How to Create the Perfect Cat-Eye Liner tutorial which shows how to achieve either a subtle or an Amy Winehouse cat eye.