On the Apple II, there are three 40 column text modes: normal (white on black), inverse (black on white) and flash. A text screen can contain a mix of text in all three of these modes.
Flash mode is roughly a 0.5s delay between cycling text between a normal and inverse mode. However, unlike the console cursor, the rate of the flash does not appear to be influenced by the speed of the CPU - my personal experience is with a RocketChip 5MHz and while the cursor flash is proportionate to the CPU speed, flashing 40-column text remains the same.
Speaking of the cursor, on the II and II+ machines it is a flashing white block - if you backspace over a character, it appears to go between inverse and normal in this 0.5s cadence. On later machines, it was replaced by a matrix of dots. So the question is not only how is flashing text implemented on the II, but perhaps an extension would be is why is the cursor affected but not the flashing characters of the text display? Is flashing text CPU-timed or something else?