Blum primes [x=3(mod 4)] zero knowledge proof? Lets say we have 2 primes, $p \equiv q \equiv 3 \pmod{4}$, and we make $n=p \times q$ public.

I can, without revealing factors, show that $n$ has two prime factors. How can i zero knowledge prove that those are Blum primes, as described before?
I believe a zero knowledge proof that $-1$ is a quadratric nonresidue would accomplish that.

If we know that $n$ has two prime factors, and that $n \equiv 1 \pmod{4}$, then $n$ is either a product of two primes both $1 \bmod 4$, or two primes both $3 \bmod 4$.

If it were the former, then $-1$ is a QR modulo $p$, and $-1$ is a QR modulo $q$, and hence $-1$ is a QR modulo $pq$.

On the other hand, if $p \equiv 3 \bmod 4$, and $-1$ is a QNR modulo $p$, and hence a QNR modulo $pq$.

As for how to implement a zero knowledge proof that $-1$ is a QNR, see this paper