I am having trouble importing a multibit private key into electrum and blockchain.com.
The presence of '/' and '+' indicate that your key is encoded in Base64
It is likely that the wallets you mention need the key to be presented in Wallet Import Format (WIF)
For example, Importing your private keys into Electrum says
Electrum supports mini private keys and full sized keys in Wallet Import Format (WIF).
also How to import keys to Blockchain.info? says
You can enter WIF (Wallet Import/Export Format) or compressed private key.
I would decode the Base64 into Hexadecimal and then follow the procedure described for WIF.
Ian Coleman's "Bitcoin Key Compression Tool" can be downloaded and run offline and might be useful. Obviously you need to be very careful choosing and using software that will be given your private key.
Is this the entire key? Or Do I extract segments from the key above? What segment?
I think that looks too long to be a 256-bit number. However, since I am not familiar with Multibit I wouldn't know what to expect and can't identify it. After running it through Base64 decoding, things might be clearer to you?