How do I get to scan QR code to get private key of a "watch only" blockchain wallet address?
It is not possible to get the private key from a Bitcoin address. This is the whole point of the cryptographic technology that Bitcoin uses.
The way that this cryptography works is through the ECDSA algorithm which uses a private key --basically a random positive integer that can't be 0 and has to be less than "N" which is an astronomical big number (close to the number of atoms in the whole universe)-- to later multiply it by a point on a cryptographic curve in a finite field (secp256k1). This process is only one way and you can always calculate the result of the multiplication, but you cannot get this result and figure out the private key through division due to the mathematical properties of finite fields.
The result of the multiplication of the private key with the point on the cryptographic curve is another point on the curve which is called the Public Key. This public key is later hashed and encoded in base58 to get the address. The hash is to add an extra layer of security, while the base58 encoding is used to make the public key human readable --this was a thing a decade ago.