Any advice for a new software leader looking to ID blind spots about their organization?

Blind spots could be multiple.   Just to name 2:

1. Knowledge of the actual applications portfolio (health, obsolescence, rationalization)
- With a TIME classification
- Identifying the jewels of the Crown system that must received more attention

2. Knowledge of the real operational costs of each and every applications
- To take better decision regarding maintenance and investments 

First, start with a Stakeholder map. Note everyone associated with value delivery in the org. Then set up time with people at all levels, intentionally looking to build trust through candid discussion. Obviously, how you approach this depends on the depth of relationships you've already established. 

There's a really interesting tool we use often called the Energy Map. It's meant to ensure shared focus among a group discussing the challenges and opportunities that exist in that team and its processes. I'd recommend considering the use of this tool with groups, both project teams and larger, cross-functional groups. 

https://verusglobal.com/energymap/