The BLM format (also known as the Rightmove V3 format) was introduced many years ago by Rightmove. As they became the biggest UK property portal it seemed to become the industry standard when exporting or importing property data. Almost any website or property portal that handles the sending or receiving of property data is likely to support BLM's.
A BLM file is essentially a flat text file that contains a delimited list of property data. It's similar to a CSV file and can be opened in a text editor such as notepad. A BLM is normally sent via FTP and is accompanied by it's associated media (images, floorplans etc). These will either all be sent separately or combined into a ZIP file.
Back in 2013 I wrote an article on the issues I see with the BLM format and how it is behind the times. Soon after posting that article Rightmove introduced their real-time feed which solved most of the issues I described and dropped support for BLM.
Other portals do still currently support it, however we expect support for it to dwindle out as other sites also move towards instant updates.