Simon, I do like the visuals of your technique, however Martin and Scott are correct that blockquotes should not contain quotation marks.
From the Chicago Manual of Style section 11.11:
[Q]uotations may be either run in … or set off from the text as block quotations, or extracts. Block quotations are not enclosed in quotation marks and always start a new line. They may be indented or set in smaller type of a different font from the text; they may have unjustified right-hand margins or less space between lines.
If blockquotes are not immediately apparent as a quotation it is the fault of the typographer or perhaps another method of quotation should be used. I agree with you that the default unstyled presentation of blockquotes is sorely lacking.
Simon, I do like the visuals of your technique, however Martin and Scott are correct that blockquotes should not contain quotation marks.
From the Chicago Manual of Style section 11.11:
If blockquotes are not immediately apparent as a quotation it is the fault of the typographer or perhaps another method of quotation should be used. I agree with you that the default unstyled presentation of blockquotes is sorely lacking.
Cheers.