hey Imjim...
I'm using slack for the XC channel (not for blocknet) and I view it with my phone or my computer.
The phone app always shows me when I login the last post but a link to "# messages since last visit" so I can always see what's going on. also when a channel, group or a name notification is happening I get a notice on my phone.
When I log in into my computer It always shows me the last post since the last visit...
No doubt, this is one of the BEST things about Slack is the mobile experience/application. I used it today while getting the car maintenance done and during my lunch meeting. Absolute wonderful experience and beats BCT, hands-down. Not even a discussion.