Normally see a black sealant that was applied with a chalking gun then spread to about 1/2" wide as mentioned
I've been using a non textured black elastomeric sealant I get at local home improvement stores, spread with a 1/2" throw away paint brush that I've cut all the bristles from (except for the last 1/4" then dipped in water or thinner.
This is applied after the floor has been painted not before