To make the black sugar syrup, add the sugars and 1 tablespoon of water into a small pot and heat until the mixture comes to a boil and the sugar has dissolved. If your sugar is not in powder form (i.e. it comes in chunks) you'll need to use more water to dissolve the sugar first and then reduce it until it forms a thick syrup. Let the syrup cool.
Please read the ingredients on your agar agar powder (it should only include agar agar). Apparently, some brands mix sugar and other ingredients into their powder which will make it set differently.