May 7 (Reuters) - Blackstone Secured Lending Fund BXSL.N reported a drop in first-quarter net investment income on Thursday, while its investment portfolio was marked at a wider discount to cost as unrealized losses increased and repayments outpaced new investments.

The business development company's investment portfolio stood at $13.94 billion at fair value as of March 31, compared with an amortized cost of $14.30 billion. That implied a fair value-to-cost ratio of about 0.98x, down from roughly 0.99x at year-end.

During the quarter, proceeds from investment sales and principal repayments totaled $450.9 million, exceeding $324.8 million of purchases, signaling a net contraction in the portfolio as exits and repayments outpaced deployment.

Net investment income fell to $178.9 million, or 77 cents per share, in the quarter ended March 31, from $188.8 million, or 83 cents per share, a year earlier.

Blackstone Secured posted $154.9 million in net unrealized depreciation for the reported quarter, compared with $43.9 million a year earlier. Net asset value per share declined to $26.26 from $26.92 at year-end, while debt remained broadly stable at about $8.03 billion.