Are "block bets" effective in practice?

You block bet when you have hands can bet and get called by worse for a small size, but tend to fold worse hands and get called/raised when they bet large.

If villain is an aggressive thinking player, you probably want some traps that block bet also so you can block bet and then call. Vs passive, less thinking players who never bluff, you can just block bet fold a lot of hands. But vs those same players, you can probably just check/fold a lot of hands on the runouts you described (flush completes, board pairs). That isn't to say maybe you would still use block bet sizing on these boards with value, particularly more marginal value or value that heavily blocks a calling range. IE diamonds complete and you have both the Ad and Kd, or you hold 3 diamonds in your hand, or you have the nuts and you just don't think villain will call pot with less than the nuts.

Vs aggressive thinking players you do have to be concerned that by block betting you aren't inducing bluffs and therefore you can't only block bet hands you are folding. Raising vs block bet can be an amazing spot to bluff, but stronger thinking players will realize this and will have traps, or even bluff catch wider than normal if they think you are overbluffing.

in the smaller plo games (for example the 1-2/5 bring in at Aria) i found them to be super effective (kind of confirms what @Mlark said, against strong thinking players in higher games more difficult).

if you have a hand with SDV, the block bet manages to accomplish 2 things at once:
you get value from a weaker hand calling you, and you protect yourself against calling to much (say, you just check, and then villain bets something that you have to call off). This "something" he is betting, you basically decide how much it will be (size of your block bet), bc if you get raised then you can easily fold

There's no set standards for block bets - it's a tool just like anything else. You can get away with these vs level 1 players as a cheap showdown, but the better players will challenge you more often then not. I think against better players block bets become a great tool to induce esp against capped ranges.

There has to be a weaker hand that will call your block bet for it to make sense. So think like top 2 pair or a set trying to get called by lower 2 pair on a board not good for your range.

You don’t have to protect this range too often, but I do so with hands that are the nuts but block the second nuts.

So for example on flush boards, if I have the A and K of the flush, I block bet. Hard to extract value from lower flushes.

On straight boards if I have the nut straight but block the second nut straight and pairs like JT96 on K9872. I block sets, 2 pairs, and the lower straight like 65.

This way you are protected against thinking players who spazz vs it seeing it as weak. That said, not too many if anyone in your average game will attack it and in order to do so properly they need the appropriate hand to do it.

If you have the nut flush blocker, with or without the second card, you would be more likely to bet big (or check). With the second nut flush blocker, now a blockbet is more appealing, because you unblock the main bluff-raising card, as well as increasing the range of hands that will call you with worse, where if you pot the second nut flush, you might be folding out all or most of the worse that will call you.

Yes.

Blockbets tend to be less useful against strong players, but very good against average and weak players.