The Black Diamond is actually the norther section of the Delaware & Lehigh (D&L) Trail which also includes the Lehigh Gorge Trail. It will eventually run north from White Haven to Mountain Top but there are sections missing. This website is current as of 2/21. If you click on the various trail sections, it will give you a description:
There is currently a gap in the trail between the Mountain Top Trailhead and Black Diamond Trailhead. About 1 mile is complete from the Mountain Top Trailhead going south before encountering the gap. From the Black Diamond Trailhead,...
delawareandlehigh.org
To get to the Black Diamond from the Lehigh Gorge White Haven trailhead, you have to ride about a quarter mile on city streets.
The first mile north of White haven up to Middleburg Rd has not yet been improved but it is easily ride-able if you avoid the potholes.
North of Middleburg Rd, There is an excellent 10 mile section open up to the Black Diamond Trailhead in Glen Summit. It is similar in condition to the Lehigh Gorge trail but is much less popular.
North of Glen Summit, there is a 4 mile undeveloped section up to Lake Road which is privately owned. From Lake Road, you can ride another 7 miles to the Mountain Top Trailhead but the trail condition is not good and hardly worth taking a detour to get there.
Right now, the Black Diamond isn't worth a trip to ride by itself but I usually include the improved 10 mile section when I'm out riding the Lehigh Gorge trail.