Viacom and AT&T remain in carriage negotiations but have failed to reach an agreement before the expiration of their current carriage deal at midnight on the East Coast. The signals of 23 Viacom networks have not gone dark yet across DirecTV and U-Verse, an outsized chunk of the pay-TV universe at 24.5 million households, but they will unless a new deal can be put in place. The high-stakes dispute follows the consequential step last week by AT&T to eliminate some Viacom…