the blank dragon balls are actually caused by a glitch in the original game from when you collect tier 2 or higher dragon balls. those dragon balls are on timed "mob style" events and just happen when they happen. that being said these events seem to not care either way weather you have a full set of dragon balls or not. basically said if you have a full set and do another event you will get a blank ball. when you get a full set just make your wish and then collect 7 again. this of course is assuming you're only collecting one tier set at a time.
that's what i did in the original game and it worked fine.
