Your air filter is black due to soot, but this is usually no cause for alarm. Burning candles can produce a buildup of soot that can make the air filter less efficient. Gas water heaters can also produce black soot, so make sure the flame is blue to indicate a clean burn. Mold is worse than soot, and it can start to grow when moisture from the air conditioner condensation is transferred to the air filter.
Mold is a health risk and can aggravate respiratory conditions, such as COPD or asthma. To prevent black filters, stop burning candles and check for carbon monoxide. If the flame blinks, there may be a problem. Make sure to check for mold in ducts and other humid areas of the house.
Regular maintenance and cleaning will help keep mold out of your HVAC equipment and ensure that it doesn't cause the air filter to turn black. Replace or clean the filter every 30 days and vacuum around the area where the filter is located. If you notice that your air filters are black every time you change them, you may need to replace them more often.