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...