God only knows what a person will find on new construction. Someone could have changed their oil. Dig out the black stuff and dump it where it will not interfere with plants and then take the soil where you dump it an use that for backfill.

Here is a link that might be useful: Propagating Perennials

After my furnace was repaired, my beautiful clematis died. It was so healthy, and had climbed up probably 15 feet, all around the porch rails, and bloomed like crazy for many years. All I could think of was to replace it and start over. When I started digging, I knew I smelled oil. Sure enough, those clueless repairmen dumped old oil in there. I made them come back and replace all that soil while I watched.