DP3,
I don't know whether or not Hammer has discontinued the BH; but am still seeing it available on several internet web sites.
Based on my experience with it, I'd say it was, in all practicality, a super-mild resin, weaker than the current acknowledged weakest resin, the Tropical Storm, nee Breeze. Officially, Hammer calls it a urethane, while Storm also calls the TS a resin.
For the last 2 seasons, thankfully, I have had no need to use the BH, but I still keep it around, just in case. I have used an older ball that is slightly stronger than the BH but weaker than my Tropical Breeze Hybrid, an AMF Orbit Extreme. The Orbit, which I got from Ebay about 2 years back, does handle a ton of dry and I use it at 4000 grit, not the stock polished finish. It also truly has a urethane type of backend even though I drilled in on the stronger side.
The Orbit has a small differential on the order of .024", if memory serves. During this times I had a Messenger Ti Pearl, but it is stronger and needs more oil than all those I've mentioned so far. For me, it required/was useful in true medium-light oil conditions.