@BejeweledOne, 

Welcome to the K2 Forum, I hope you find it a friendly environement. 

To answer your questions, I would need to know what you are using to build your workflow.  Are you using K2 Designer, K2 for Sharepoint etc?

As far as Active Directory VS Azure Active Directory, I am really not sure. All I've read about Azure Active Directory is about authentication. 

For K2 Designer there is an Active Directory Wizard

When I need to create something with IF logic I like to use Smartforms. 

"In my workflow I need to query AD to grab some user details IF a field in the list item is blank."

I would probably transfer data from a SmartObject to data labels or text boxes

I would set up a rule 

You need a new activity. From your previous activity, drag a line to the new activity and set the line rule to do the IF check. Put a smartobject event in there and call you smartobject.

As for querying the AD, there's 2 options,

1. Use the Active Directory Service 2 (AD Service2) smartobjects. You might need to create the smartobjects first. (This is for OnPrem ADs)

2. Use the AAD smartobjects you mentioned. This is.. obviously for Azure AD. 

Could you point me to any documentation on creating the AD2 smart objects?

I would be very grateful for any assistance. 

Thanks

This can usually be registered using K2 Workspace or the Smartobject Service Tester tool (C:Program Files(x86)K2 blackpearlBin):

http://help.k2.com/onlinehelp/k2blackpearl/userguide/current/webframe.html#AD03.html

You can right click on the ADUser Service Object to create a new Smartobject from the SmartObject Tester Tool; please give it a new/relevant name.