And now as per present WhatsApp issued OTA update to its app: Blue tick marks indication itself gives the confirmation as exactly when the message has been read by the receiver. Here the most important thing to notice is that: as these changes or updates are made OTA which in a sense: that these changes or updates will not be found or available on app stores.
And in-case if the user has a WhatsApp installed is the latest version then automatically user may find or notice the blue check marks as the instant feedbacks or status updates. And if at all the user has not such marks means that it has to be supposed that the update is staggered & it will be updated within a short period.
About WhatsApp:
WhatsApp Messenger is a proprietary, cross-platform instant messaging subscription service for smartphones and selected feature phones that uses the internet for communication. In addition to text messaging, users can send each other images, video, and audio media messages as well as their location using integrated mapping features.
WhatsApp Inc. was founded in 2009 by Brian Acton and Jan Koum, both former employees of Yahoo!. The company is based in Mountain View, California and employs 55 people It is currently in the process of takeover after Facebook announced its acquisition of WhatsApp Inc. on February 19, 2014, for US $19 billion. Read more on Wikipedia
Textual content in this news article (about WhatsApp) is taken from Wikipedia which is licensed / available under Creative Commons Attribution-ShareAlike License.