Cheaper oil should help economy? Wrong: BlackRock

When will Fed hike rates?
When will Fed hike rates?   

BlackRock bond guru Jeff Rosenberg blames 2015's weak economic growth on the collapse in oil.

Many economists and investors have been waiting for the benefits of lower crude, but it hasn't happened so far, the fixed income strategist said Wednesday on CNBC's "Squawk Box."

"The collapse in rig counts shows up in the economic data as a collapse in investment data."

Rosenberg spoke before the government reported that first-quarter gross domestic product grew at a lower-than-expected 0.2 percent. The economy advanced 2.2 percent in the fourth quarter and 5 percent in the third quarter.

Read MoreUS economy stalls as weather, low energy prices bite

A detailed review earlier this month by CNBC of 30 years of the government's GDP data suggests a longstanding trend of lackluster first-quarter growth.

Due to economic weakness, Rosenberg said, he expects the Federal Reserve to start hiking interest rates for the first time in nearly a decade in September. At the beginning of the year, he had pegged June for liftoff.

The Fed is scheduled to release its policy statement Wednesday afternoon, following its two-day April meeting. Rosenberg does not believe policymakers will box themselves in to any timetable for rate hikes. The economy should starting picking up in April and May, he predicted. "Then you're going to pull forward some of these Fed expectations."

He also pointed out the Fed isn't bound to signal changes in monetary policy around its scheduled meetings. The next scheduled meeting of policymakers is mid-June, with economic projections released and a news conference held by central bank chief Janet Yellen.

There are many Fed speeches and other events between now and then, Rosenberg said, recalling the spark for the so-called "taper tantrum" over the possible scaling back of quantitative easing bond purchases came in May 2013, when then-Fed Chairman Ben Bernanke was appearing before Congress.

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