I just pulled up the release notes from those two links above just fine.
Plan is for B230M1's to stay supported under 2.1 infrastructure. Meaning the blade will stay at 2.0(5a) and the Infrastructure (UCSM, FI's, IOMs) can be at version 2.1 and be fully supported. I'm verifying now.
I was also curious to get the answer to this question, so I checked the interoperability matrix and the Release Notes and yes, ESXi 5.1 is support on UCSM 2.1 (infrastructure) with the blade running 2.0(5) firmware.
Select B-Series, UCS Release 2.0(5), Server Model: B230-M1, OS-Vendor: Vmware, OS: you'll have the option to select ESXi 5.1, which means it's supported. If you try with 2.1 or 2.0(4), ESXi 5.1 is not an option in the drop down menu.