The moisture regain of the blend is calculated as the weighted average of the moisture regain of polyester and cotton:
\[
\text{Moisture regain of blend} = (0.7 \times \text{Moisture regain of polyester}) + (0.3 \times \text{Moisture regain of cotton}).
\]
Substitute the given values:
\[
\text{Moisture regain of blend} = (0.7 \times 0.4) + (0.3 \times 8.5) = 0.28 + 2.55 = 2.83 \, %.
\]
Final Answer:
\[
\boxed{2.83}.
\]