I vacation in Delaware pretty much once a year.
In DE, they have a CCDW (Concealed Carry of a Deadly Weapon) law that encompasses many types of weapons- handguns, knives, other weapons. Carrying a concealed knife is illegal, UNLESS the knife is a "common pocket knife", with a blade less than three inches. Additionally, while carrying a sub-3inch "common pocket knife with a folding blade" concealed, the blade must be in the closed position. Per the letter of DE state law, you can open carry (to carry visibly) (OC) a knife that doesn't meet that sub-3" common pocket knife description (you may also OC a number of other types of deadly weapons without need for any permit- such as a handgun), provided that such carry does not violate town or county ordinance.
Delaware is unusually clear in their carry laws. However, one must be especially careful if they choose to Open Carry a Deadly Weapon in Delaware. Open Carry is legal on the basis that it is not prohibited or regulated by law. Unfortunately, that also means that there is no statuatory definition of what constitutes it being visible, which would result in an LEO haveing to use discretion as whether to charge you with CCDW when you thought that you were OC'ing. Additionally, if the deadly weapon was obscured or hidden from plain view in any way (including by the vehicle itself), it could and most definetely would constitute CCDW. You also need to make sure agarment (like a coat) doesn't accidentally cover the knife- you could land yourself a CCDW rap if that happens.
To avoid any problems, when I am in Delaware I ONLY carry folding knives that have blades less than 3" by AKTI measurement standards.
Also, there is a law in DE prohibiting switchblade sale or posession. (it is a one line all encompassing prohibition in the deadly weapon dealer licensure statute, that applies to all persons licensed to sell deadly weapons or not.
If you do have interest in OC'ing, you should talk to local law enforcement to see what they consider concealed both on your person walking about, and also in a vehicle.
Some towns, such as Dover, have town ordinances that are much stricter than state law. In Dover, for example, it would be necesary to have a CCDW license to carry any knife and not run afoul of Section 14-5, City of Dover Code of Ordinances.
Here is the state statutes, scanned from ATF Publication 5300.5 State Laws and Published Ordinances- Firearms.