Sorry for the necro, but I wanted to post here because the search results are high.
While I can confirm that the Post Process blend weight doesn’t work properly, you CAN adjust the properties of a post process volume in realtime by getting the settings and then “Set members in Post Process Settings”.
Search for the value you want to adjust (exposure for example) and you’ll see an option to check a bool for “As Pin”. This means you can use a timeline to adjust the value over time giving you the ability to essentially adjust the blend speed. It’s convoluted as heck, but it’s at least doable.
