The trail up Black Butte is 5.2-ish miles (roundtrip) with an elevation gain of about 2000 feet. It’s rocky, but nicely graded. From the top, you’ll have incredible views: Mount Shasta to east, Castle Crags to the south and Mount Eddy to the west. You can also see a snowy Cascade up in Oregon, 70 miles to the north. Access to the trailhead is along a gravel road. It may be snowbound in the winter.
For more detailed directions, check out this link.
For more details about this trip, check out the Black Butte Norcalhiker post.