Until now the BBM app had been limited to Blackberry's own devices
The Blackberry Messenger (BBM) app is to be offered as a download to run on rival platforms.
Blackberry said it would initially offer texts, photo messages and group sharing functions on devices running Google's Android and Apple's iOS operating systems from "the summer".
It added it planned to roll out screen sharing, voice and video calls - all without charge - later in the year.
The move could prove disruptive to Skype, Whatsapp and other rivals.
Blackberry chief executive Thorsten Heins revealed the surprise news at the end of his presentation at the firm's annual developers conference in Orlando, Florida.
The company said that more than 60 million Blackberry owners already used BBM at least once a month.
But Mr Heins played down the idea that offering the feature to rival devices would harm sales of the Canadian company's own handsets.
"You might ask the question why is Blackberry doing this now," said Mr Heins.
"It's a statement of confidence. The Blackberry 10 platform is so strong and the response has been so good that we are confident the time is right for Blackberry Messenger to become an independent multiplatform messaging solution."
He added that for the app to work iPhone users would need at least iOS 6 and Android users the Ice Cream Sandwich version of Google's software. There was no mention of Windows Phone.
One analyst said it was too soon to know if the move would force competitors to change their own strategies.
"BBM has been a significant traffic driver for Blackberry - particularly the consumer audience," said Chris Green, principal technology analyst at Davies Murphy Group Europe.
"Expanding it to be multiplatform not only widens the consumer appeal but also may help woo back corporate customers it lost due to earlier technical problems.
"However, other players like Whatsapp will only drop their fees if they see BBM making major inroads into other platforms."
Ben Wood from telecoms consultancy CCS Insight added that Blackberry might have felt forced into the move because of the growing popularity of cross-platform alternatives which also includes Facebook Messenger.
"It will increase feature competition among mobile messaging platforms," he said.
"It dramatically increases BBM's long-term relevance - but it is unclear how Blackberry will ensure the move benefits its own hardware sales."
Blackberry also announced a new smartphone powered by its BB10 system - the first to be targeted at emerging markets.
The Q5 features a physical Qwerty keyboard and a 3.1in (7.9cm) touchscreen.
The Blackberry 10 device will be released in parts of Europe, Asia, Africa and Latin America in July.
It should be cheaper than the Z10 and Q10 which were unveiled in January.
However, the firm has yet to confirm pricing.
"We understand the importance of having devices to suit all markets and needs," said Mr Heins. "It's a sleek, slim high-performance device."
One company watcher said it was important for Blackberry to have a device running BB10 on sale in parts of the world where its handsets were still bestsellers.
"The Q5 could be a very significant device for the company because there is a significant opportunity for high-quality low-cost smartphones," said Adam Leach, devices analyst at telecoms consultancy Ovum.
"If Blackberry can replicate the success of the Blackberry Curve in emerging markets then it will certainly help establish the Blackberry 10 platform.
"However, Blackberry has significant competition in this area with low-cost Android devices and with Nokia's Asha 501 which sells for just under $100 [£65]. It will be crucial to see if Blackberry can match or undercut that."
Ovum predicts that emerging markets will account for 40% of smartphone shipments by 2017.
According to data from IDC - another tech consultancy - Blackberry devices accounted for just over 19% of global smartphone shipments at the start of 2010. But the firm suggests that figure had dropped to less than 4% by the end of last year.
Blackberry has yet to release sales figures for its first BB10 devices.