A form of digital signature where the signer is not privy to the content
of the message is called a:

A.
Encrypted signature

B.
Zero knowledge proof

C.
Masked signature

D.
Blind signature

Explanation:
A blind signature algorithm for the message M uses a blinding
factor, f; a modulus m; the private key, s, of the signer and the public
key, q, of the signer. The sender, who generates f and knows q,
presents the message to the signer in the form:
Mf q (mod m)
Thus, the message is not in a form readable by the signer since the
signer does not know f. The signer signs Mf q (mod m) with his/her
private key, returning
(Mf q)s (mod m)
This factor can be reduced to fMs (mod m) since s and q are
inverses of each other. The sender then divides fMs (mod m) by the
blinding factor, f, to obtain
Ms (mod m)
Ms (mod m) is, therefore, the message, M, signed with the private
key, s, of the signer.
Answer Zero knowledge proof refers to a zero knowledge proof. In general, a zero
knowledge proof involves a person, A, trying to prove that he/she
knows something, S, to another person, B, without revealing S or
anything about S. Answers Masked signature and Encrypted signature are distracters.