The investment giant Blackrock is stocking up in shares of former punters' favourite Gulf Keystone Petroleum PLC (LON:GKP). A regulatory filing shows the American group on Wednesday passed the 5% ownership threshold. While the last year has seen stock in the Kurdistan-focused oiler come rise 100p to 250p, over recent week the price has become range-bound. The Blackrock investment could be seen as third-party validation for the company, which two-and-a-half years ago underwent a painful refinancing. More recently it has been focusing on achieving output of 55,000 barrels of oil a day - impressive for a company valued at just over £500mln. However GKP receives payments direct from the local government of Kurdistan, which is in the northern corner of Iraq, and therefore is drip fed the cash. Also, it has to accept a fairly hefty discount for its output - around US$21 a barrel less than the prevailing Brent crude price. However, with oil creeping above US$70 a barrel the 'net back' (profit on production) should be rising significantly, even with the financial harnesses holding Gulf Keystone back.