Need advise on water hoses for blk water & Sani Flush sysyem.

Yes you can. It's your camper, you can use it any way you want.

Your black tank flusher has an in line check valve that prevents black water from coming "back" up into the hose. So, you can use any hose on the flusher.

Yes, you can fill your black tank with the flusher system too. As stated above, the reason is discouraged is because of getting distracted and filling your tank and backing up inside your camper and flooding it. But yes, you can do it if you want. It won't hurt anything.