It's because Black Fox got released as a movie that especially the story is suffering. Many questions remain unanswered which, yes, is normal, because the story doesn't find an end in the movie. However even then some things should have been further explained or resolved but you can feel how 3Hz Studio just didn't have the time to do so. Had the anime been released as a series the story would have had much more potential but like this it is decent at best.
The Art and Sound are good, the soundtrack isn't something to remember but it still suits well to the scenes and I liked the design of the characters. The overall artstyle of the anime is also solid but nothing unique.
As for the characters it's hard to say a lot. Here again, with the time they had the characters are well written and all got their own backstory, even if the movie isn't going to deep into those, because, well, time and stuff.
In the end I think Black Fox offers quite a bit. It has a nice change between serious and homorous scenes and I can't deny that despite the flaws the story has I really enjoyed the movie, from the start till the end. So I can only recommend you to give it a try, even if it doesn't have the best ratings on MAL.
