How to import a 64-byte long private key Private keys are 256 bits (32-bytes) long, according to bitcoin wiki:
https://en.bitcoin.it/wiki/Private_key

Diamond Circle, an Australian startup, sent me a pair of bitcoin address and its private key. Blockchain.info shows that the money is still in the given address.

The problem is that the private key is 64-bytes long. It's base-64 encoded evident by the trailing ==

Since it's not in WIF, I wonder how to import it. Has anyone seen 64bit private keys before?
Taking into account that 64bytes keys are probably seed keys in bip32 format you might need to derive the key first. I modified the code to reflect that 

npm install coinstring hdkey
node 
var HDKey = require('hdkey')
var cs = require('coinstring')

// This is your base64 key 
var seed = 'oMQqnDrGq/K6aplGroOvGPUb8cn6fazEySUTzE3QFYNDQcd13NTA+sc1R8VmLYGp6TYaCqxgSnOjIb2RA7zorw=='
var hdkey = HDKey.fromMasterSeed(new Buffer(seed, 'base64'))

var hex = hdkey.privateKey
var version = 0x80; //Bitcoin private key
console.log(cs.encode(hex, version)) 


It should give you the familiar WIF format private key.

5HyfJgfUm7WVgUrLuo9uwtJR6ME7MYfGRhK2aeckZRooSKYGXP8