Better to maintain a list in datastore or csv file?
As I know get_by_id() (small) operations are free of charge. So you will pay only for instance hours. But I think it would be better to store subscription emails in another kind, because storage is cheap, and denormalization of data is a good practice on GAE. Anyway CSV does not looks like a good idea.
I want to maintain an email blacklist of people who don't want to ever receive email from my service. 
Before I send each email, I want to do a lookup on whether the recipient is in the list.
Which of 2 choices is better? 

I can create a BlacklistEmail model in datastore and key it on the email address so I can have faster lookups using get_by_id(). In 99% of cases, the recipient will not be in the blacklist so this would actually cost a read as it would not hit memcache.
I can store the blacklisted email in a csv file and check whether the recipient is in the list. This seems like it wouldn't cost anything, but I am not sure about performance. I don't expect the list to be very big.
Any other better way?

Which is better in terms of cost and performance?