Security proof of partially blind signature (schnorr signature based)
It follows by the probing strategy/heavy-row lemma, which traces back to [Oka,FFS]. Read §6 in this paper for a nicer explanation. 

References:
[Oka]: Okamoto. Provably Secure and Practical Identification Schemes and Corresponding Signature Schemes. CRYPTO 1992.
[FFS]: Feige, Fiat and Shamir. Zero-knowledge proofs of identity. JoC 1988.