Interesting to see villain folding this turn, since his flop floating range will be a lot of pocket pairs, diamonds and spades BDFD. Perhaps he folds his offsuit A highs on the turn, although most of the A highs have showdown value vs your betting range + 2 overs. River will be an interesting decision on non diamonds, because 87dd doesn't block too much of his folding range, since he shoudlnt have many 7x and 8x flushdraws. On a blank (like a low heart) you probably bet TT+ for value and your most natural bluffs will be some suited Diamond and Spade connectors, both seem fine candidates to me. You have to be careful with high cards, like a K for example, because on such river card your value range becomes narrower (since you probably check back most AK on the turn) and its easy to end up overbluffing.

why did you bet so small on that flop?

not anymore :D