In no particular order, here are my thoughts:

• most languages need to reform in large ways due to being at least partially tonal/inflection based (such as a rise in inflection to indicate a question). All languages need to add an additional conveyance mechanism to account for the loss of tone and inflection to indicate feeling, and anything else speech patterns normally convey, as suddenly all language works like texting
• people suddenly suck at talking in exactly the same ways they suck at writing, because they have to pick the right conjunction or homophone. Good luck two us all
• directional blinder hats would be a thing almost immediately (something that shields the light but has a covered hole in front to selectively open however wide you need, probably with hand controls and color filters and shit)
• light pollution dies down to facilitate conversation, but dark sky areas have to shutter their projects due to conversational twinkling (sad outcome :( )
• indoor light gets dimmer to facilitate conversation. 60 more stubbed toes happen every month
• sunglasses become the new unplugged headphones
• someone develops filtering goggles that cut the specific human communication wavelengths for people with epilepsy. They are a big hit with commuters and parents
• since people can no longer talk but it sounds like all else remains the same, someone would develop a translation device that does “blinks to speech” for blind and epileptic people, who could maintain use of the auditory old language and still function fine in society (good outcome, yay!) (I considered how they communicate back, but there’s no reason their light thing wouldn’t work so this is fine)
• more people opt to have their outer eyelids removed so they can eavesdrop on conversation while looking like they are sleeping (weird outcome, but it is a surgery that exists and divers sometimes elect to get it done to avoid wearing a mask. Inner eyelid is left in place, can’t tell from outside, but you can see through it apparently)
• private conversations become much more difficult, which forces everyone to act nicer in public, which reduces the amount of time people can be shitty, which in turn makes everyone nicer (yay happy outcomes!)
• it gets a lot harder to hide that you are watching porn or kinky boning when the blinking light gives away the… dialogue. Blackout curtain and door light stopper sales skyrocket literally overnight
• people rarely go missing in the woods or in wrecks. Everyone has a beacon every night, and there are huge social awareness campaigns to use your light this way. An international help pulse is developed so no matter where a person gets lost, they can blink for aid. 1km is quite far, meaning they would light up the area around them, especially multiple people blinking in unison. A project is launched to have satellites scan the night-facing surface looking for the pulse pattern, and is wildly successful. There is a brief trend among young teens to cry wolf, until the bills for wasting global resources start rolling in
• lots of famous people suddenly find themselves jobless, as singing is no longer a career. Since light pulses are completely sensually unrelated to music, instrumental music makes a big comeback, as do poetry recitations and stage plays. All the weirdo instruments from over the course of history are resurrected (best attempt) to add variety to the cultural landscape once filled with voices
• television and movies lose cultural significance as they lose the ability to tell many of the stories they do now (blinky light gives away your location in horror, ruins ambiance for romance, interrupts action sequences, etc. it’s just not amazing for the current form of visual entertainment)
• translation becomes a lot easier, as the effects of accents and dialects diminish. The light pulses can easily be read by software and translated to a different pattern (human speech sounds are so so much harder to parse)
• people have an easier time learning other languages now that everyone shares the same blinks framework; no pronunciation difficulties, just new patterns
• animals mostly very much dislike humans, and find us quite alarming. The blinking doesn’t help. Animals trained to respond to verbal cues have to be retrained to understand the blinks are an attempt to communicate something to them. Many animals now have problems in their homes due to the change (very sad outcome :( )