This is a geomantic saying. Blue smoke is a blue gas, an auspicious gas and a sign of good luck. According to the past, you can be an official. Now many people don't have ancestral graves, so they are not allowed.
Another solution is that when you make a fire, blue smoke will come out, indicating that you will "fire". According to Chen Shuaifo, chairman of the China Yijing Association, in the book Eye of the Earth, it is a common phenomenon that the smoke emitted from ancestral graves is caused by the earth's atmosphere, and the low-melting gas or solid in the soil produces colored gas under the action of temperature, which overflows the surface and forms blue or white tiny particles, that is, smoke.
The original intention is to have a good thing, or to be a big official. However, this is also irony and abuse. For example, if someone becomes a team leader and someone denounces it, they will say that someone has a ancestral grave.
It is best not to put firewood and hay on the grave, because the old grave will produce combustible gas, especially before and after Tomb-Sweeping Day, the climate in the north is dry, and burning paper on the grave will increase the temperature, which is likely to make the grave burn, and people who don't know will panic.