Having a block qoute markup is a good idea and at least in Germany one important criteria for an accessible site. Besides a block qoute markup it would be also nice to have the inline version <q> </q> (see Inline Quote Proposal).
Note: blockqoute mustn't be used for indentation.
For accessibility it is always a good idea to use semantic markup (<strong>) instead of normal markup (<b>). Code elements could be marked with <code> and so on.
Here in Germany also these things must be met: