Female. Males do not have the hourglass.
Relevancy: http://entomology.wsu.edu/insectoft.....lackwidow.html

Males can have the hourglass as well actually.

The most compelling feature for determining spider gender is the development of the pedipalps.

As you can see in this photo, the pedipalps are very well developed with a almost a boxing glove type look.

The photo you linked to shows the underside of a female, but the dorsal side of the male, thus you would not see an hourglass if it were present.

The males can have a mark but it's generally really really orange or yellow - maybe the color is just off in this shot - you do have a fine point about the pedipalps.... those are waaaay too large for a female - though I've neeeever seen a male with such a defined hourglass - it's usually more dot like. Whether male or female it's a pretty unique specimen.