Sorry, I missed your post. As long as there is a turning of color near the blossom end, you can save it. So yes, save the seed from the reddish side of the tomato for seed saving.
~~Tam~ You can bury all your troubles by digging in the dirt.
Gardening in March
Gardening in April
Gardening in May
Shop at Amazon and Support AGF
Are you shopping online? Click this link first and A Gardeners Forum will receive a commission for your
referral at Amazon.com (shopping through this link to Amazon will not have any impact on your prices at Amazon).