The goal of the paleo diet is to eat the same foods that were available to people living during the Paleolithic era. The thought being that our bodies have not adequately adapted to the over processed foods that exist today, and so it wholly rejects any foods that resulted from farming and modern technology. A good rule to follow is anything that can be hunted and gathered is okay, which translates to lean meats, fish, fruits, vegetables, nuts, and seeds.
Cocoa, and even chocolate, definitely fit within the bounds of paleo. The tricky part is processed sugars and milk do not. And the majority of commercial chocolate, including micro-batch craft chocolate, contains processed sugars. A general rule of thumb among Paleo dieters is the darker the chocolate, the better. Go for dark chocolate above 70% and even better above 85% cacao content. The more cacao % in a chocolate bar, the less sugar and other ingredients it will contain.
If you're hesitant to try chocolate that is above 85% cacao because you find it unpalatable or too bitter, do yourself a favor and try some high quality craft chocolate before you rule out all ultra dark chocolate. Unlike industrial mass produced chocolate, dark chocolate from craft chocolate makers have a myriad of complex flavors and is both rich and delicious.
Alternatively, you can also look for chocolate bars that don't contain any cane sugar, and are sweetened with natural sweeteners. The more popular paleo alternatives for sugar are honey, maple syrup, maple sugar, and coconut sugar. Another great option is to use dried fruits like dates and raisins to sweeten things up.