Does Django Block When Celery Queue Fills?
It's impossible to really answer your question without an in-depth analysis of your actual code AND benchmark protocol, and while having some working experience with Python, Django and Celery I wouldn't be able to do such an in-depth analysis. Now there are a couple very obvious points : 

if your workers are running on the same computer as your Django instance, they will compete with Django process(es) for CPU, RAM and IO. 
if the benchmark "client" is also running on the same computer then you have a  "heisenbench" case - bombing a server with 100s of HTTP request per second also uses a serious amount of resources...

To make a long story short: concurrent / parallel programming won't give you more processing power, it will only allow you to (more or less) easily scale horizontally.
I'm doing some metric analysis on on my web app, which makes extensive use of celery. I have one metric which measures the full trip from a post_save signal through a celery task (which itself calls a number of different celery tasks) to the end of that task. I've been hitting the server with up to 100 requests in 5 seconds. 
What I find interesting is that when I hit the server with hundreds of requests (which entails thousands of celery worker processes being queued), the time it takes for the trip from post save to the end of the main celery task increases significantly, even though I never do any additional database calls, and none of the celery tasks should be blocking the main task. 
Could the fact that there are so many celery tasks in the queue when I make a bunch of requests really quickly be slowing down the logic in my post_save function and main celery task? That is, could the processing associated with getting the sub-tasks that the main celery task creates onto a crowded queue be having a significant impact on the time it takes to reach the end of the main celery task?