causing a surge of oil to blow through valve seals? Is that even a possibility? Might explain the plume and current smoke if the seals were damaged.
In the all important quest for lightness, uncle Colin left out the valve seals.....
Being serious, there aren't any. You just have the cam bucket over the valve stem/spring and then a cast iron valve guide. So you'll always get a touch of oil going down there and burning, unlike more modern engines which might have seals ahead of the guides.
I watched the video and couldn't make my mind up. Like BDA I thought it more whitish than dense blue, but it's a difficult thing to photograph and get the colours on screen exactly as you're seeing them in real life. My car will produce smoke like that before it gets up to temperature and even at temperature if I let it idle and then rev it up to 3 or 4k then you'll see smoke. I don't think it's a sign of impending doom (fingers crossed ! ) because it's always done that, in fact they both do. It's easy to compare with a modern engine and think it's falling apart, but these were designed before emissions and are going to burn some oil.
The engine sounded all right on the video so here's hoping there's no real crisis. As John says, a hot compression test is probably the next useful step, but if you're getting 140 cold and more importantly within 5-10psi between cylinders then I think you're all right there. 110-120 or a big drop on one or two adjacent cylinders is when I'd start to wonder.
Valve guides I do know about because I did them on the Elan. Before I'd stop at traffic lights for a minute and there would be a diesel engine cloud behind me when I took off. Also coasting downhill on a trailing throttle tended to suck oil in and when you put your foot down you'd see a puff in the rear view mirror. The car ran ok though and it was only embarrassment at the looks I got from other drivers which pushed the job to the top of the list.
Brian