To get full body Haki in Blox Fruits, you need to start by unlocking Haki. Head to the Middle Island in the first sea, and find the green-roofed house behind the big mansion where the aura editor is located. You can also locate an aura editor on any island.
Once you have unlocked Haki, you need to level it up. Using a fighting style like Dark Step helps as it covers your legs in Haki first, then your arms. Each hit you land earns you 1 XP for Haki, so keep grinding and hitting to level up. Alternatively, using a sword with Haki enabled can speed up the process.
For the fastest methods, you can try the AFK method. Go to Starter Island, set a spawn point, and let Bandits attack you if your health is above 1000. Another non-AFK method is to use a non-logia fruit and let enemies hit you or use a logia fruit and a sword to hit enemies without taking damage.
Gathering NPCs in areas with many enemies also helps gain Haki experience quickly. Use the weakest sword to do less damage and get more hits. If you have a fruit like Buddha, it makes AFK grinding easier. If not, use a logia SL Elemental fruit to avoid taking damage from enemies.
By following these steps and methods, you will quickly achieve full body Haki in Blox Fruits. Keep hitting and grinding, and soon you’ll see your Haki covering your entire body.