There was some thought behind that decision. B and S (for Blockshares or blockShares) were out because they were used in NBT and NSR addresses and we didn’t want confusion between the projects. From there, we decided a number prefix might feel familiar to Bitcoin users who mostly used addresses beginning with “1”.
In my research I couldn’t find any major crypto projects using “8”, which I thought was strange because it’s considered a very lucky number in China, where people will pay extra to have an 8 in their phone number or house address. It wasn’t a coincidence the Beijing Olympics started on August 8, 2008 (08/08/08) for example.
Given the popularity of Bitcoin trading in China, 8 seemed like as good a number as any for BlockShares addresses. I think BlockShares addresses will occasionally start with 9 (such as multisig addresses) but @sigmike would have to confirm.