Ok, yes, the OM Blackhawk you got is sorta like an old Colt. Direct contact between the hammer & firing pin when the hammer is at rest. If you slightly pull back the hammer (to the very first "click") the hammer will be off the firing pin, but still not safe. A sharp blow, or even dropped can shear off the notch on the hammer to allow the hammer direct contact with the primer of a live round. So, to properly carry an OM, you open the gate, go to half cock, rotate & load one round, skip one chamber, then load 4 more. Close the gate, go to full cock, then carefully lower the hammer by holding it as you pull the trigger. Of course, do so while pointing it in a safe direction. Never lower the hammer from the half cock position. It can cause a live round to be under the hammer, AND it will give you that dreaded "turn ring" on the cylinder.
The alloy g/f is normal. Likely it's marked; "XR3-RED." Yes, you can swap it out for another factory Ruger g/f easily. The one caveat is that any of the NM grip frames will not have the same trigger return spring set-up. But, do not despair. If you get a NM g/f, and want to use it, you can make (easily) an adapter to fit in the NM g/f to allow the OM trigger return spring to work properly. Do a search here & you will find the adapter I'm talking of.
Brass g/f's of Ruger vintage will be just fine, w/o mods. Aftermarkets,,, maybe, maybe not.
The screw hole pattern on the Rugers hasn't changed in the SA's ever. The Ruger factory brass g/f is marked; MR3-DB. But be prepared to pay about $300 for just the g/f. Aftermarket brass g/f's are cheaper,,, but some can have issues in fitting. Also, the Ruger one will take the Super Blackhawk grip panels. You may also want the wider SBH trigger to match, and an OM Super trigger will cost around $50. The slot in the g/f is for the wider trigger, but the standard BH trigger will work.