Does Geomancy able to outcast black magic? This depend on how the master does your home Geomancy (Fengshui). In order to give you a clean ideal. We must understand what is black magic? Every race or culture has black magic. Different practitioner has different level of black magic. A lot of people always thought that black magic is original from Thailand. However, this is not true. Black magic original from Cambodia. Cambodia used to be a very big country which include Thai, Vietnam, Laos, Mymmar, Malaysia. It is later the split that divide into various country. How do black magic comes from? Majority thru Buddhism. Old people always thought that black magic comes from Journey to the west Tang Seng (唐僧）where he collect the sutra back and where crossing the river he meet an accident and the sutra got wet. So he find a big stone to dry his sutra and mark are left on the stone. Because the sutra ink on the stone is reverse side hence that is how the black magic form. This is totally wrong story. In Sutra we consist of two volume upper volume（小乘） and lower volume （大乘）. Upper volume stated the way how Buddha practise before enlightenment and lower volume is the way to Bodhisattva after he enlightenment. This form the difference Northern （北方） and Southern （南方） Buddhism. Northern is practising Theravada and Southern practised Mahayana. That why you can see that Theravada practise pray only to Buddha (Shakyamuni). They pray to Deva, Lersi. Where Mahayana pray to other Buddha e.g Amituofo and Bodhisattva e.g. Goddness of Mercy. Of course at this era due to technology people don’t different them out anymore. So where is the black magic? The most that you see on movie is from Theravada. It is the ways where Buddha learn from Lersi before he become enlightenment. Before Buddha become Buddha he also have teacher. Don’t be so naive that Buddha don’t seek for teacher before he become Buddha. Every master has a teacher, it just like everyone has parent. Did you see anyone born out of rocks?
How do Geomancy / (Fengshui) prevent black magic. First we must understand that when we are born we are protected by seven layers. (Mothers, Ancestor, Mother God, Teacher, Deva, Guidance god and Buddha). Hence, before the black magic works on you, it must be able to pass thru the seven layers. What Geomancy did is to enforce the seven layers. For me, I will choose a good spot to place religious statue which include your name and eight character. Is this breakable. To be honest, yes. If the practitioner does have the keys to open the seven layers. He / she must close the light on you – you are down on your luck. Then he / she will ask the seven layers to give way so black magic cannot enter to your body. This is spiritual black magic which is very common see in movie. If thru consumption of food which is Gu（蛊) then Geomancy won’t be able to help you much it just blame that you stick of offended to the wrong person. If you have a strong guidance from your guidance gods then it will help to in preventing getting involve with that person.
You question in mind is do I have the keys in open the seven layers. To be honest with you I don’t have. All this is passed down knowledge from my past master. Which I pen down in my blog. So that people get benefit from it. Not my intention of hurting or harming anyone.