So, with Nozomi's reply, its the macrophages that gradually absorb the dye, thus producing the splotchy areas next to the vessel walls.
To further this topic, we have a note from Egle regarding the SIZE of the macrophages:
"Yes, macrophages are the largest blood cells I think!
"Here's some pics for scale:
(monocytes are precursors for macrophages)
Those above are human cells -- a human macrophage is about 21 micrometers, while a human capillary is about 5-10 micrometers. Those proportions are probably similar in mice too!
(Monocytes, at 20 micrometers, are comparable to the size of macrophages, (Latin: "big mouth") at 21 micrometers and vary greatly in their names and morphology according to where they reside, whether in the lungs [as alveolar macrophages], liver [Kupffer cells], placenta [Hofbauer cells], kidneys [intraglomerular mesangial cells], brain [microglia], etc.
[Note by Plenum per Wikipedia information. ])
Okay-y-y... Well, for me, two take-aways... First, I'm truly surprised at the size of the macrophages., and want to read up on what other sources have to say about the cell. Second, the algorithm, still not perfect, blacks portions of the video, but has not learned to black out the macrophages. This explains the blacked out portions we've seen in the last few weeks.
"Mystery of the White Splotch" seems solved... Dye absorbed by Macrophages.