They're all proprietary. Each application uses their own algorithms to convert the raw data. To the best of my knowledge none of the adjustments made with one application will translate identically in another application unless both applications use the same raw conversion engine "under the hood" (e.g. Adobe Lightroom and Adobe Photoshop both use Adobe Camera Raw to do the actual raw conversion). So you have to choose only one application to do raw conversion.
If you then want to use other tools to further edit the image you must export the results of the raw conversion in a standardized raster image format that can be edited by the other tools. Probably the most common way to deal with this is to do the raw conversion and then export the images as 16-bit tiff files.
Of course converting to tiff locks in a lot of the decisions made in raw conversion: black point, white point, gamma correction, etc. and reduces the amount of further adjustment to things such as color temperature/white balance, etc. But the 16-bit tiff has a lot more information than, say, an 8-bit jpeg. The maximum number of gradations in an 8-bit color value are 256. The maximum number of gradations in a 16-bit color value are 65,536. In addition to the lower bit depth there is also the consideration of compression. So the tiff file will allow a lot more adjustment before things start to fall apart than a jpeg will. The biggest disadvantage of 16-bit tiff files is their size: A raw file from a 20MP camera will run around 24-30MB. The exact size depends on content. Most raw files are compressed using lossless compression so the more variation there is in the scene the larger the file size will be. The same 20MP image converted to a 16-bit tiff file will run around 100MB uncompressed.
From the comments:
After posting the question last night, I discovered that darktable supposedly (and to some extent) can read the adjustments inside adobe's xmp files - for what it's worth, here and here are the links. I haven't managed to make it work thus far though.
Even when the instructions can be read properly the question still remains, is darktable applying the exact same algorithm to get the exact same result? Or is it just using the darktable algorithms to approximate the results from a certain setting using ACR? My hunch is that you could get the exact same results by editing in darktable to begin with as you could get by translating the instructions from ACR using darktable. You just have to learn how to use the different GUI to get there.
Indeed @MichaelClark, one could use LR, Br or darktable without needing to use another! I was more interested in finding a workflow, that is futureproof - in the sense that I won't be locked down in one particular application/ecosystem/OS.
In that case always save your original raw files. That's about as future proof as it gets. People like to make a lot of noise about a "standard" raw format, but it's just a bunch of smoke and mirrors. DNG is no more future proof than .cr2 or .nef. And more applications let you convert .cr2 and .nef files than let you work with .dng files.
Nothing is future proof. Absolutely nothing.