Thank you for your submission.  The pictures are out of focus; consequently, we will be unable to use this as an official record for this species being in Delaware.  The picture appears to be of a caterpillar, but we are unable to determine which species it may be.  We need to have a physical specimen to verify what this particular animal is.  A specimen of this  animal can be dropped off at one of the extension offices or the plant diagnostic clinic and follow sample submission by filling out a form available at  the link below.

clinic and sample submission

https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/plant-diagnostic-clinic/

cooperative extension office locations

https://www.udel.edu/academics/colleges/canr/cooperative-extension/directory-contacts/

I don't think what you found is a a black velvet slug.  I think it is a caterpillar.  No the slug you mention is not common in Delaware.