BL Black Diamond Coating SEALANT for eyelash extensions BL Black Diamond Sealant is one of the best aftercare eyelash extension products on the market. For best results, use this sealant daily to extend the life of your eyelash extensions. This coating eyelash treatment protects both the glue bond of the eyelash extension from moisture and pollutants, whilst adding a shimmering wet look to the eyes. Application is recommended after treatment.