Thank you Tristan for such as terrific question.
We elaborate on this topic in our blockchain white paper: “cisco.com/go/blockchain”, titled “Blockchain by Cisco” – page 12, section 4 Security and Analytics.

Unlike public blockchains, private blockchains are established among permissioned “known” enterprise consortium partners. For example, across a set of enterprises participating in a supply chain. Each enterprise hosts or manages a blockchain node. Nodes are typically not exposed to the public internet and are located behind each enterprises' firewall (often within the highly secure enterprise demilitarized zone DMZ).

Thus, a 51% attack actually requires:
1. The respective enterprises within the consortium to collude against each other, using their respective nodes, at an enormous scale. Let’s say we have 10 enterprises participating in a supply chain. We would need six out of the ten enterprises to agree to “band together” to collude against the remaining four in order for a 51% attack to occur.

2. A massive scale external assault.
A 51% attack could occur, in theory, without collusion, if six out of the ten nodes were compromised simultaneously by external bad actors. Penetrating one enterprise's defenses to take control of a node is difficult enough, let alone penetrating six enterprises simultaneously (especially if they are using Cisco security).

Lastly, if there were such a high degree of collusion or external attack, the blockchain network would exhibit clear telltale patterns or signatures within the traffic and transaction patterns.

In our whitepaper, we define the BAF – Blockchain Application Firewall (similar in concept to a web application firewall), which uses Cisco’s expertise in analytics and machine learning to detect for any unusual traffic signatures or signs of collusion in the event of such a scenario.

You can download our whitepaper at Cisco.com/go/blockchain.