Hi, 

Any type of systemic herbicide can help dissolve them but it depends if you choose a post emergent selective herbicide or non-selective herbicide. The active ingredients in the link provided for post emergent broad leaf weeds should help without effecting the health of the grass. 

Alternatively you can carefully paint on a non-selective like glyphosate or triclopyr onto the leaves and it may kill some of the grass surrounding them. 

You can also hand dig or pull what you can or continue to mow them and eventually they will exhaust their root storage and stop returning. This could take a while since Black Locust are known to sucker and be prolific. 

Let us know if you have further questions. 

Emily