You mention that the Font properties support this so I did a little digging in reflector and found this interesting:
TextBlock
public static readonly DependencyProperty FontSizeProperty = DependencyProperty.RegisterCoreProperty(0x40003714, typeof(double));
Control
public static readonly DependencyProperty FontSizeProperty = DependencyProperty.RegisterCoreProperty(0x80003714, typeof(double));
Inline
public static readonly DependencyProperty FontSizeProperty = DependencyProperty.RegisterCoreProperty(0x3714, typeof(double));
So all three of those classes define a FontSizeProperty and the id used look very similar as if that first bit is just a flag of some kind. So perhaps there is something going on under the covers that allows what you're calling property inheritance since the the framework might transfer the values between these similar dp ids.
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