If I want to store a substantial number of Bitcoins, would there be any add security or safety to doing this in multiple blockchain.info hybrid ewallets, in stead of all in one?
For large numbers of coins, software wallets (online or desktop), no matter how well designed, are not safe. Online clients aren't safe because the ones that are encrypted your side (like blockchain) might somewhat be susceptible to man-in-the-middle attacks. As Lohoris mentioned, Blockchain uses js verification with Github, requiring both websites to be compromised - of which there is a slim chance. Standard desktop clients are inherently not safe either, not when you're storing thousands of $/£ in coins. Even with all the safeguards in place, the absolute safest storage method is to keep the keys offline.
You would be much better off generating a large number of keys using an offline computer and storing them on encrypted flash drives in safe locations. This is what the biggest known BTC hoarders (Winklevoss) do.
You can then add the addresses without the keys to Blockchain for easy monitoring and zero risk.