What a wonderful large scale fabric! I've been trying to appreciate and use large scale prints for something other than backs and it can be a stumbling block on what to do with a fabric you truly love. Black works perfectly with it, I love using black as a neutral despite all the pet hair in our household.
Personally, I like the narrowing sashing. Wider is also ok. Maybe do a group of four blocks together with narrow sashing and then wider sashing between the groups of four... The purple border will be great, I'd probably quilt in a similar flower into it.