First post here. I should mention I am a director with a VFX background, so whilst I know about codecs not an expert in finishing and archival.
However, a master is the best possible copy of your final film from which all others are made. For shorts, for me this usually means Prores 444 HQ - does the job for a 2k copy if shot on a compressed format.
Once you've done your first high quality render to a lossless format, multiple compressed renders can then be done much quicker.
For a feature film at Hollywood level, it might be a 10bit DPX sequence or 16 bit EXR (massive file sizes for an entire film).
Bearing in mind a DCP print for cinema is essentially just a JPEG 2000 sequence averaging about 1.3 mb/frame @ 2k (from memory).
For your purposes I would recommend considering rendering to H.264 with a data rate around 10mb/s - because Blu-rays tend to be rendered to this wrapper. I have had some success with Prores LT which is a very efficient but high quality format. I would recommend several renders; one for saving your work for future prints and one for the purpose of your Blu-ray master. A web render is going to need to be even more compressed.
Depending on your Blu-ray software, I believe its possible to give it a file and tell it to encode and fit it to the disc. This will be easier if you render your film out in scenes rather than one massive file.
Hope it helps,