Answer:
No one knows who Hitler paternal (father's side) grandfather is. So people could blackmail him.
Step-by-step explanation:
Since even Hitler himself doesn't know who his paternal grandfather was he couldn't prove the fact that he was purely "aryan". Since he couldn't prove who his grandfather was, multiple rumors spread about who it could be. One of the most popular theories is that his grandfather is Johann Georg Hiedler or his younger brother Johann Nepomuk Hiedler.
Another rumour includes the fact that his grandfather was jewish 19-year-old (hence making him non-aryan), with the name of Leopold Frankenberger