According to Wikipedia, "Blue Marsh was the name of the village that was located where the lake now is." Completed in 1979 and commissioned under the Flood Control Act of 1962, the lake is now home to 36 miles of trail and 1,100 acres of water popular among boaters, fisherman, swimmers, and more.

One of the main trailheads for the loop is the dam on the southern end (shown here). Another popular access point is the large unpaved trailhead at Church Road. From there, you could hike the northern half of the lake as a loop, instead of an out-and-back. The shallow, calm waters of the northern half of the lake also make a popular area to kayak, paddleboard, canoe, or even fish. Several boat launches also offer large parking areas and facilities, but at a fee.

Long in length, the trail itself is never too challenging. The terrain includes a mixture of rocks, roots, and abandoned roads, far less challenging that most of the trails in central Pennsylvania. In perspective, the rugged Appalachian Trail even passes through just a short drive to the north. As with most trails along a reservoir, the soil do not drain quickly and is the area is prone to flooding. After any amount of rain, give the trail plenty of time to dry out.

Compared to the eastern side of the lake, the the western side of the loop is significantly quieter. You'll explore open meadows and farmland, with plenty of blazes to show you the way. The entire loop is easy to follow and hard to get lost.

Sources: https://en.wikipedia.org/wiki/BlueMarshNationalRecreationArea