Actually, if you rely on the definition Storage is part of IaaS since by using Azure Storage you ara isolated from the HW layer and you can directly use storage services for your infrastructure
https://azure.microsoft.com/en-us/resources/cloud-computing-dictionary/what-is-paas/
But also it can count as PaaS sometimes since PaaS services cover IaaS to provide more functionality to its users.
So its more related with your scenarios if you attach your storage to a VM it behaves as IaaS
but if you want to use additional features that Azure provides it can be thought as PaaS