I had some black sesame butter, I think it was labeled, and it was too hard to get around the color. One of those foods you have to taste with eyes closed. And same with the whole seeds!

the shiny white hulled ones will give you a pleasant color, while using the dull looking unhulled ones will probably take more time to grind up into a smooth paste (& potentially be a touch less smooth on the tongue) but will yield a deeper earthy flavor to the tahini. (never tried it with the uber flavorful black sesame seeds, not a fan of grey tahini!)

There are also options of roasted or raw seeds.

Most tahini is made with hulled seeds. Hulled seeds contain about half the calcium as unhulled seeds. You can find more info here: http://www.whfoods.com/genpage.php?tname=foodspice&dbid=84