ARCore requires android-24 or above and the ARCore plugin in Unreal 4.18 is still the developer preview version, it only works with supported phone for now.(Descripción general de ARCore y entornos de desarrollo compatibles | Google for Developers)
You are seeing a black screen probably because ARCore isn’t supported on the devices you were testing. You can query if ARCore is supported using UGoogleARCoreSessionFunctionLibrary::IsGoogleARCoreSupported() function.