Valve stem seals
Blue smoke on the first start of the day that lasts only a few seconds is valve stem seals leaking a little. These are very hard-wearing plastic grommets around the valve stems at the top inside the cylinder head. Upon installation each valve seal is pressed down onto a boss inside the cylinder head. The idea is to do everything possible to prevent oil from seeping into the combustion chamber by way of the valve stem guides.
In your situation, a little of the oil standing in your cylinder head after shutting down has a chance overnight to seep into the combustion chamber past your valve stem seals and down the valve guides, so when you start in the morning that oil burns blue. It soon stops because not that much oil seeps in.
It happened to me on a professionally rebuilt engine. I think it happened because there was an engine vacucm line close to the battery, which was weepy, and the acid attacked the vacuum line allowing acid fumes to be drawn into the cylinder head.