Blocked Process Report -two updates causing a long block
The Blocked process report (which you have) can be a little confusing at first. There are a few important sections

Blocking-Process
This is the process(SP, query etc) that is causing the blocking. The process is using a resource that is required by the Blocked-Process

Blocked-Process
This is the process(SP, query etc) that is blocked and is waiting on a a resource that is in use by the Blocking-Process.

waitresource
This details the resource that the process is waiting on. Taking your example of 
waitresource="OBJECT: 99:774293818:0 "

The 99 refers to the Database ID of your data and the 774293818 refers to the table. Using the ID 774293818 and systables you should be able to find the table that is causing the blocking. 

I would also set up an Extended Events Trace to capture the parameters of the offending blocking process and review the execution plans.

A good starting point is https://support.microsoft.com/en-us/kb/224453