There are now three main approaches up Black Mountain, from north to south:

Monte Bello Open Space (Page Mill Rd exit)

Rhus Ridge trailhead of Rancho San Antonio Open Space (El Monte Road exit), also reachable from Hidden Villa except in summer months

PG&E trailhead of Rancho San Antonio County Park (Foothill exit)

Of the three, the PG&E trail has by far the most parking but is probably the hardest approach and is definitely the longest. It's also prone to mud in the winter and there's not much shade in the summer. Plus you'll be walking directly under high voltage transmission lines, which make an eerie sound and might not be that aesthetic. On the upside, some of the highest views over the mid-Peninsula, not too many other hikers, and a good challenging training hike are available to the hardy walker of this route.

This trail is so new it isn't even on the maps yet. Follow the PG&E trail on the map here:

http://www.openspace.org/preserves/maps/pr_rancho_san_antonio.pdf

from the equestrian parking lot until it terminates at the Vista Point. There is now a well-maintained although very steep 1.1 mile connector trail, newly dubbed the Quarry Trail, from that point to the Black Mountain Trail, under the care of the Open Space District per this agreement:

http://www.openspace.org/CGI-BIN/agendas_minutes/2011.11.10.BlackMtnEasement.pdf

You can hike this as an out-and-back, or a lollipop loop from the Vista Point, or even a full loop down Black Mountain and around via the Chamise Trail.