Can I get free PS+ games 'retroactively'?

Unfortunately, there is no way to retroactively redeem past PS Plus games.

If you haven't redeemed those games when they were free, you'll have to either purchase them or wait until they're offered for free once more.

To redeem a PS Plus game, simply "purchase" it while it is still free. You don't need to actually download the game.

You cannot claim free games from past months. For instance, it's April now, meaning if I didn't visit the store and download Bloodborne through PS+ last month then I can't go back and do so now, even if my subscription is still active.

However, if you already claimed a free game in the month it was offered, then wait a few months and remove it from your system, you can go back and re-download it for free as long as you're still subscribed to PS+