Do mainstream economic models use the blank-slate view of humans? Human capital is an important part of GDP forecasting. Do the models in use today view humans as "blank slate", meaning they can be educated to the same level, or do they take into account heritability of human abilities?
Standard economic treatments of skill generally assume that people have some baseline level of skill (human capital) which varies over people, and also that skill (human capital) can be improved via education or other methods.

Most of the time, models don't specify why baseline skill varies over individuals, but heritability would presumably be one reason. There are certainly some generational or evolutionary models that assume that parent and child baseline skill levels are correlated, which is in effect a heritability assumption.

As for GDP forecasting specifically, most GDP forecasting methods focus more on education level as the primary (or only) measure of human capital (both the average level and its distribution), but this is likely because baseline skill can't be measured very well, and can be proxied pretty effectively by education anyway because of selection effects.

So, in short, no, economic models aren't blank-slate, although using education as the only measure of human capital, which is sometimes done for pragmatic reasons, does look a tiny bit like a blank-slate assumption.