If you have been bagging Black Hag, then swing by and bag the similarly named Blackhaggs Rigg as part of your jaunt. At 452m it's over 100m shorter than it's pal, but will still get your legs warm on the climb up.
From the parking at Hethpool follow the valley floor south for 3km until you pick up the permissive path which doubles round on itself and then heads up to the little hillfort.
Then just follow the track up to the top of Blackhaggs Rigg.
Then just keep heading up the track to the top. Turn around and you will have a canny view of the Blackhaggs Rigg Hillfort.
Follow the route above or find it on OS Maps. 5km to get there with 366m of climbing.