I did it the other way once, at least 15 years ago. Was still in my teens or early 20s at the time.
My memory of the Ennerdale side was bog and heather with no actual path on the ground.
The drop down to nether beck was I think just steep grass with no path. The path along nether beck was for me at the time largely unrideable (on a 2004 Cannondale Jekyll, so reasonably capable bike, not sure about the rider).
By the time I got down to wastwater I was defeated, too tired to ride and losing the daylight so I had to phone my mum and get her to come down from Whitehaven and pick me up. She was not impressed.
Perhaps riding up nether beck on a better bike with better riding ability might be doable. I’ve never bothered trying that bridleway again in either direction.
To get from wasdale to Ennerdale, what I have done before is follow the BW from near Windsor farm (just north of nether wasdale) through blengale forest, then scalderscew wood. Cross worm gill (sketchy stepping stones), up tongue how (muddy fields at first but gets better) out to the cold fell road. Head north then take the BW up Blakeley rise. Peel off left either by the transmitter (there’s a path marked on the map but I don’t recall it being much more than a vague sheep trod) or the later path heading more directly northwards to the top of crag fell. Take the path towards ben gill and follow till you get down to the valley, then take the BW following the western shore of Ennerdale and carry on till blacksail hut.
It’s a hell of a long way but the vast majority is rideable.
If you really want to do blacksail and you’re up to it, the 4 passes route is a lot better day out in my opinion, though involves some challenging riding.
