What they are and when to use them.
For those of you who have never used or heard of a bloodline, a bloodline is exactly what it sounds like. It is the technique of outlining without any ink. You use just water to lubricate the needle. When you do a bloodline it leaves a red bloodline for the time that you are doing that tattoo, and then goes away when the tattoo heals. If you are doing a large piece then you may want to use what is called a greyline.
A greyline is when you mix 3-5% black ink with the water so that it will leave only a faint grey line for future reference to color or shade off of.
Bloodlines are used when you don't want a noticeable black outline. This gives you a chance to pull off certain special effects such as soft smoke, background effects that you don't want to overpower your foreground, negative space and geometric shapes.
When you shade or color off these bloodlines they will no longer be noticeable.