I’m getting my car registered, and I’d like a black license plate to match the paint. Nevada has a Las Vegas Raiders License plate that is black, but it has the Raiders' logo. Would it be legal to cover the logo in, for example, black duct tape?
SCOTUS had the case of Wooley v. Maynard, 430 U.S. 705 (1977). Wooly took the tape and covered up the NH state slogan with tape. The state claimed that was illegal, Wooly won the case in SCOTUS: the state may not compel speech:
Held: 2. The State may not constitutionally require an individual to participate in the dissemination of an ideological message by displaying it on his private property in a manner and for the express purpose that it be observed and read by the public.
(a) New Hampshire's statute, by forcing an individual, as part of his daily life -- indeed, constantly while his automobile is in public view -- to be an instrument for advocating public adherence to an ideological point of view he finds unacceptable, "invades the sphere of intellect and spirit which it is the purpose of the First Amendment . . . to reserve from all official control," Board of Education v. Barnette, 319 U. S. 624, 319 U. S. 642. Pp. 430 U. S. 714-715.
(b) The State's claimed interests in requiring display of the state motto on license plates (1) so as to facilitate the identification of passenger vehicles, and (2) so as to promote appreciation of history, individualism, and state pride, are not sufficiently compelling to justify infringement of appellees' First Amendment rights. The purpose of the first interest could be achieved by less drastic means, and the second interest cannot outweigh an individual's First Amendment right to avoid becoming the courier for the State's ideological message. Pp. 430 U. S. 715-717.
On the other side, Summers v. Adams, 669 F. Supp. 2d 637 (D.S.C. 2009), banned a state from producing a religious number plate - it would advance a religion, which is barred for state speech due to the Establishment Clause. This case showed that number plates are state speech. That plates are government speech was also strengthened in Walker v. Texas Division, Sons of Confederate Veterans, 576 U.S. 200 (2015), which held that specialty number plates are government speech, and people can't force the state to carry a specific type of plate with speech the state doesn't like.
While Wooley speaks for the prospect that covering up the compelled government speech is not banned, and Summers as well as Walker hold all specialty number plates to be government speech, this does not mean that all number plates are compelled speech. There is no SCOTUS (or lower) case that makes an optional specialty plate compelled, especially since the option of standard plates with other or no message exists.
In Wooley v. Maynard, a New Hampshire man was sentenced to jail for covering up the "Live Free or Die" slogan on his New Hampshire license plates. He sued the state to bar it from enforcing the state law that prohibits drivers from obscuring any letters in their license plates.
The Supreme Court reversed the conviction, holding that the First Amendment generally protects drivers from being forced to carry government messages that they do not approve of:
The State is seeking to communicate to others an official view as to proper appreciation of history, state pride, and individualism. Of course, the State may legitimately pursue such interests in any number of ways. However, where the State's interest is to disseminate an ideology, no matter how acceptable to some, such interest cannot outweigh an individual's First Amendment right to avoid becoming the courier for such message.
It seems clear that the court did not mean that drivers may obscure the alphanumeric code uniquely assigned to each license plate, but its reasoning should apply to the Raiders logo.
You may not put foreign materials on the license plate. That being said, it is hard to imagine that someone would ever be ticketed for covering the sports team logo only on a license plate, unless it is done as "giving you a break" from a more severe offense.
NRS 482.275 License plates: Display.
4. Every license plate must at all times be securely fastened to the vehicle to which it is assigned so as to prevent the plate from swinging and at a height not less than 12 inches from the ground, measuring from the bottom of such plate, in a place and position to be clearly visible, and must be maintained free from foreign materials and in a condition to be clearly legible.