How to import a 64-byte long private key
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