Another ULA effect...
I remember that I saw it on some title screen of an old ZX game, can't remember where it was... Nevertheless, it's another not emulated (yet) ZX feature. This effect generates blinking thin lines, color of the line is defined by next non-flashing ink color. See below:
Notice that width is much smaller than standard pixel and it's blinking. You can play with color too, simply change INK at line 45.