Sharkskin is a truer black color, but is harder to apply ESP if you have drier lids. It works best on those with oily lids.
Blackground is much easier to apply, doesn't tug on the lids, but is not as "convenient" as a shadestick. It is a dark grey-black.
If you want a true black color underneath, try blackground fluidline as a base- works really nicely! But it is REALLY black! If you use this though, you need to let set before packing on e/s- but boy, does it make colors pop! If you're not comfortable with precise application of bases on the lid, go with p/p instead of f/l.
HTH hun