Captain Blackheart is a famous pirate, most known for his skill in drinking endless amounts of whiskey.
The Captain spends most of his time in the Pirate's Plunder Tavern in Buccaneer's Den with his favorite drink, 'Captain Blackheart's Whiskey'. This Whiskey is sometimes a key ingredient in Grizelda the Hag's magic brew.
To get the Captain to part with some of his whiskey, you will have to prove to him that you have what it takes to be a pirate. Doing that involves a lot of alcohol and some new clothes.
First you will need to find yourself a Tricorne Hat and any other clothes you think fits a pirate, the hat is the important part though.
Once you are all dressed up, you go buy a lot of alcohol from the tavernkeeper and start drinking...
After about 5 pitchers, maybe more maybe less, talk to the Captain again and he will let you know what he thinks of you. If he still thinks you are a useless landlubber, then you will have to drink more, but if he thinks you have proven yourself a true pirate, he will hand you a mug of his prized whiskey.
You can then continue on your quest for the ingredients to Grizelda's brew.
Note: Captain Blackheart will only talk to you if you are on Grizelda's quest and the whiskey is listed in her recipe.