Several years ago when I was newer to cycling I exploited my local knowledge of the area and devised the safest route to and from Blackwood.
Okay, "In my humble opinion"
Very few other cyclists use it but it would probably cause less aggravation if people did.
Basically, riding from Murray's hill road to Blackwood up Diosma drive cyclists are a pita. The climb is steep, there is no where to overtake due to limited vision and traffic coming the other way.
Winns road however. Also steep, cars have very good vision around cyclists and traffic is much lower.
To get to Winns road, along Main road, cars have more safe options to overtake, the road is wider and its much easier to be out of their way.
Going the other way, Corromandel parade down Diosma drive is fast for cyclists. We aren't a pita as its not that hard to approach reasonable speed (careful of hairpins turns and roundabouts), then the climb through Murray's hill road allows many more overtaking options than does the alternate route (Main Road) and traffic is lighter.
From the bridge at the bottom of Murrays Hill Rd is a paved path and a small bridge linking to McNamara Rd and on to Main Rd. Much of Main Rd has a parallel shared path although the area around the Coromandel Valley Institute requires a couple of road crossings. Not perhaps ideal road bike territory but good for MTB/hybrid riders like me. I did an exploratory ride down that way recently (from Crafers) and entered the Craigburn Farm area from next to the bridge and ended up hitting the plains via the back of Flinders Uni.
Also, that footbridge (which crosses the Sturt R. at McNamara Ford) near Horner's Bridge (bottom of Murray's Hill Rd) links to Light Rd, then out to the top of MHR by Lyric Pl. Good alternative to MHR in both directions. To avoid the steep part of Diosma Dr, one could continue on Craiglee Dr and then up Protea Ave, or use Coromandel Pde all the way.
(I often start or finish rides at a point along the ridge of Coromandel/Diosma/Protea.)
An MTB enthusiast could ride up Horners Corners and then into Craigburn Farm Circuit >Fergusson Ave >The Farm Lane. (It's a nice walk with my son's dog.)