I see this in the documentation:

Status: The next column in the table is the Status column. Lines display an icon indicating whether the test succeeded (e.g. the site was up) or failed (was down) when last checked, indicated by an upwards- or downwards-pointing triangle. The downward triangle is always bright red (). But for successful tests, the color of the triangle indicates additional information. If the triangle is a shade of green (), the site has changed recently - specifically more recently than it had an error. If it is a shade of orange (), the test recently failed, but has since recovered - again specifically more recently than any change. The shade hints at how recent the change or error occurred, with it starting off bright green or orange, and eventually fading to gray as time passes. While a test is being checked, the status displays a spinning progress indicator, or if you don't have an internet connection at the time of the check, it shows a flashing lightning bolt until the connection is restored. This column is included by default

But two of my monitored sites (which are up) are showing a black triangle. What does that mean?