Blockbuster Entertainment Co. yesterday announced a 51% gain in net income for the three months ended Sept. 30 to $ 41.3 million (22 cents per share) from $ 27.4 million (16 cents) for the like period last year.Third-quarter revenue was up 24% to $ 283.8 million compared to $ 229.7 million for the like period a year ago. Net income for the first nine months of 1992 grew 50% to $ 96.9 million (53 cents), vs. $ 64.6 million (39 cents) for the similar period in 1991. Nine-months revenue up Revenue for the first nine months was $ 802.3 million, a 30% increase from the $ 616.6 million reported for the first nine months of 1991. “The growth of our net income is attributable to market share gains as well as to our efforts to increase operating efficiencies, achieve productivity gains and reduce overhead expenses,” said Blockbuster topper H. Wayne Huizenga in a statement. O’seas biz growing He also added that international business has been growing for the 2,989 -store system, which includes operations in nine foreign countries. Standard & Poor’s and Moody’s Investor Service rewarded the company a two-level upgrade on credit ratings for its liquid yield option notes during the third quarter. Moreover, the company recently negotiated a new revolving credit facility that, among other things, increased its committed borrowing capacity from $ 200 million to $ 325 million and contains more favorable terms and conditions than a prior facility.