Randomize your blocks with this simple class. The blocks will stay the same until the server reloads/restarts

Randomizer.java

package de.fel1x.randomizer;

import org.bukkit.Bukkit;
import org.bukkit.Material;
import org.bukkit.block.Block;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.BlockBreakEvent;
import org.bukkit.inventory.ItemStack;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

/*
* Plugin by fel1x
* Tested for versions 1.8.x to 1.15.x
*/

public final class Randomizer extends JavaPlugin implements Listener {

    private Map<Material, Material> randomizedBlocks;

    @Override
    public void onEnable() {

        List<Material> normalBlocks = Arrays.stream(Material.values()).filter(Material::isBlock).collect(Collectors.toList());
        List<Material> randomizedBlocksList = Arrays.stream(Material.values()).filter(Material::isBlock).collect(Collectors.toList());

        // Shuffle randomized blocks
        Collections.shuffle(randomizedBlocksList);

        // Put blocks in HashMap
        randomizedBlocks = IntStream.range(0, normalBlocks.size()).boxed().collect(Collectors.toMap(normalBlocks::get, randomizedBlocksList::get));

        PluginManager pluginManager = Bukkit.getPluginManager();
        pluginManager.registerEvents(this, this);

    }

    @EventHandler
    public void on(BlockBreakEvent event) {

        Block block = event.getBlock();

        if(block.getType().equals(Material.AIR)) return;

        block.getDrops().clear();

        Material blockToDrop = randomizedBlocks.get(block.getType());
        block.getWorld().dropItemNaturally(block.getLocation(), new ItemStack(blockToDrop));

    }
}