I changed my fuel filter the other day and it was black.when I changed it back in March it was the same way but I had just got the truck so I just thought it hadn't been changed in a while but now I think it might be something else.Would fungis cause this problem?

 

Could be algae if it's slimy. If it's kind of sticky it could be asphaltene. Overall it's a pretty common occurrence and usually not a problem.

 

Algae. Found in bio blended fuels. Most commonly ground in above ground fuel storage tanks. Are you buying fuel at a station or do you you have your own bulk fuel storage? Talk to your fuel supplier about what you are finding. There are algaecides fuel additives that can be added to prevent this from happening.