it is impossible to buy a black grenade in horde mode. no marker is placed on it (too many items in the zone)
#purchase problems
amber forum
keen copper
I haven't tried reproducing this (rarely play horde mode personally, not even sure which map this is), but this sounds like it might be something for #1027556185156628570 instead of tech support.
wise stratus
this is on the map Grad, you can still buy it but you need to angle it right. it is annoying due so i really think they need to fix it.