I think it would be best if you go ahead and get a blocked account just to be safe. I have heard of instances where account statements of students from the US and Canada that had the required amount (8640 euros) were rejected by the Ausländerbehörde.
Such students were asked to switch from a normal account to a blocked account to have their visas extended. The reason for this is the Ausländerbehörde realized some students had lots of people loan them money, showed a fat balance to the authorities, and then transferred all of the borrowed money back once they had their visas extended. They are therefore very cautious now in accepting only bank statements.