i like your runner- the echo quilting would probably be nice and your borders too- i (personally) would not use black thread- i would use white- black is going to show up every single stitch---of course if that's what you like/want then by all means go for it- it just wouldn't be my choice-
maybe put together a small practice sandwich & do what you want in the borders on it with the black- and decide if you like it or not before commiting yourself on the real thing....just my thoughts.