You should probably be tracking your blockers separately from your flow diagram. The flow diagram will only highlight a large group of blockers well after they've become a problem. So, my first answer is that you should already know where your blockers are. You shouldn't depend on the flow diagram for that.
To answer your question directly, you can tell the difference by looking at the band after the bulge. E.g., if your workflow has design followed by development and you see a bulge in design.
- If the bulge us caused by a lot of blockers, the development stage will be starved of work and that band will narrow.
- If the bulge is caused by a bottleneck in development, the development band won't narrow (and will probably widen a bit).
You can also look at how long individual items are sitting in the design column.
- If the cause is a lot of blockers, some items will have exceptionally large 'time in column' values while others move quickly through this stage.
- If the cause is a bottleneck in development, the 'time in column' value for all tickets will rise together.