Apple Health tracks the data. It just doesn't provide graphs except for the most common variables. I'm not really sure why no one makes a graphing app that works off HealthKit data
<sightly less cynical view> There's been persistent rumours for a few years now that Apple are working on blood glucose sensing for Apple Watch. If those rumours are right (and a big part of me really hopes they are) then that'd likely come with built in apps that destroy every blood glucose related 3rd party app.
Edit: I just noticed the original post wasn't specifically blood _glucose_ related. My own "blood test" requirements made me jump to an erroneous conclusion...
https://apps.apple.com/ca/app/heart-reports/id1448243870
No affiliation with the app, just a user.
You are right that you cannot enter the data yourself, as far as I know. But if your lab or physician uses a system that can let you download the records, they will be stored. So I have access to all my blood labs, but there is no easy way to graph them.
As far as I know, there is no App Store rule against making an app that lets you log the variables manually, nor is there a rule against making an app that graphs stuff. Keep in mind that there are a lot of Apple rules around HealthKit data to protect privacy, and writing such an app could be difficult.