cant SWIY get some naphtha? what about sodium carbonate?
black and goey sounds like possibly some of the basic acqueous solution came to the final product.
Add just a pinch of sodium carbonate to maybe 100ml of water (ph 9-10). Then redissolve the goey product in a solvent that is not miscible in water, mix in a container the solvent and the sodium carbonate water, you can shake no problem. then let separate and pull out the solvent, and evaporate or freeze precipitate to get the crystals (if there are any).
Or alternatively (or even doing both) you can recrystalize, which is to redissolve everything in a minimum amount of warm naphtha/hexane/heptane. Only the dmt will be pulled to it, and some junk will probably remain undissolved, you can throw that away and freeze precipitate the naphtha