Hi Patrick,
The BLM files are simply a delimited file format where each field is seperated by a special character e.g. comma, pipe, tilde. Essentially it's just a text file.
The .BLM extension is just used to help id the files on Rightmove's FTP servers.
Most of the major estate agency software suppliers (e.g. DezRez) have built in Rightmove export options where they will generate the necessary file and supply it to Rightmove's servers automatically. Rightmove call these 'datafeeds'.
If you are building a datafeed from scratch, you just need to generate the file in accordance with Rightmove's datafeed specification. If your site is delivering properties from a database it should be relatively straight forward, just a matter of exporting the fields in the right order with valid items in them.
You get the spec and further advice from their Automated Datafeed (ADF) team on 0845 338 0076.