As far as I know, isn't a sex berry supposed to change your pet's gender for sure? I've fed my pet Blackwall two of them now and neither has changed their gender from female to male. I'm not sure whether I'm remembering how they work incorrectly or if it's an actual bug. Any help would be greatly appreciated.

You have to choose use on. Not feed.

Awkward. Ha ha, thanks for clearing that up. I really need to pay more attention when using things. I totally missed the use on option.

No problem! The use option is for the effects of the sex berry - what you're aiming for - and the feed is just so it counts towards the gourmand / total food eaten achievements. :)