You must first understand that cancer is caused by a weakened immune system and a systemic fungal overgrowth.
Fungal associated cancers are extremely rare. And it is even rarer for fungi to go systemic. When fungi, such as Aspergillus niger, induce cancer it is from the toxins they release. In the case of of A. niger they produce aflatoxins that can cause liver cancer in people with severely compromised immune systems.
I discussed the claim about Candida causing cancer before: