Why does coffee taste burnt?

There are a couple reasons why coffee can taste burnt:

ROASTING 
Many roasters overdevelop (burn) beans during roasting. This could be because the beans don't have inherent natural flavors so the roaster is adding "flavor" from the the roasting machine. It could also be because the roaster isn't as skilled.


Beans that are black, really dark brown or oily on the outside are overdeveloped and will taste burnt. There is nothing you can do during brewing to change that taste.

If the coffee description includes words like "smokey" or "French roast", that also could be an indication that the beans are burnt.

BREWING
Brewing improperly can also result in burnt coffee so it's important to pay attention to water temperature and brew time. 

1) Water Temperature. Water that is too hot for a particular brew method can result in burnt coffee. For a drip machine, the ideal brew temperature is 206-208 whereas for espresso the ideal brew temperature is 198-201.  So if you brew espresso at 208 or drip coffee at 212, you may find burnt coffee.

2) Brew Time. If coffee is brewed too long, it will taste burnt. Note that brew times vary based on the brew method and the coffee dose.