My 1990 S-10 did the same thing and it was a bad ABS solenoid. The flashing means it is giving you the code - apparently that happens when you leave the fault alone long enough (though I don't think it is supposed to). I fixed mine by unplugging the ABS computer since the rear wheel ABS never worked all that well anyway - a little black tape around the connection (not plugged in) keeps it close to the computer and clean and still passes inspection. There are codes for the ABS faults, but the easiest way to see if it is the ABS is to first unplug the harness and if the light goes off and stays off you have your answer, now if you want to fix it you have to read the codes from the flashing light. I cannot recall the codes off hand but I think they were in my factory service manual.
Bryan Lundquist
Bangor, ME
1998 Suburban 2500 4X4 with optimizer 6500 engine (4/07) with battery modification, remote mount FSD cooler (behind bumper), 285/75/16LRE BFG All Terrains (going to 4" backspacing wheel required 1.25" lift), AFE air cleaner, Stewart Warner Maximum Performance EGT, Boost and Trans temp gauges, 3->4" stainless exhaust, Rancho RS9000xl shocks, LEDs in all exterior lights.
2005 2500HD crew cab 6'6"bed with 6.0L no mods yet.
2010 2500 Suburban - couldn't wait any longer for a duramax.
Also 1978 Chevy Blazer 350cid HD engine with Fisher plow; 1988 Suburban