Native to the American Southwest and northern Mexico, Opuntia macrocentra is a black spined cacti that forms patches 4 feet high and several feet across. In full sun the pads take on a purple hue and some consider it a variety of O. violaceae. Flowers are yellow and orange and are produced in spring and early summer.
     Got a small piece of this rooting and plan to plant it in the ground in spring. The other pictures were taken of a patch of it in front of the Sabbar Shrine in Tucson. Also, Xenomorf, a fellow member of the Davesgarden.com gardening forums, has some nice closeups he's allowed me to use. Thanks, guy!

A large mature patch

Closer view of the flowers

Flower by Xeno

And another view