Here is my take on it. The smoke from the coals below passes through the hot fire/ coals above and reburns which then produces thin blue smoke.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-73, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle