Citing unnamed sources, Fortune says the firm could be investing as much as $300 million into Jawbone.
Earlier this week, a separate report from Re/code also suggested Google may be considering a "strategic" investment in Jawbone as well, but didn't mention a specific number.
The company has raised more than $400 million in its 16 years of existence, according to Fortune, which means a $300 million investment from BlackRock would bring its total funding up to around $700 million.
The report also comes as Jawbone has been facing trouble producing its next-generation fitness band, the UP3.
The company unveiled the device in November, and it was originally supposed to ship in time for the holiday season. Jawbone pushed its release date back to 2015, but it still won't reveal a more specific time frame.
A Jawbone spokesperson also recently told TechCrunch that the company is having issues with production. When Business Insider asked for more information regarding the UP3's release in January, Jawbone didn't offer comment beyond the fact the company is targeting an early 2015 launch.
Jawbone has declined to comment on both reports from Fortune and Re/code.