Most of Bangladeshis are by Pakistani standard much darker but not Blackers.
There are about 10% Bangladeshis who are descendants from Afghans (Pathans), Turks, Arabs, Persians and Kurds. They are 'converted' Bengalis. Most of them reside in Sylhet district of Bangladesh. Rest of them have spread all over Bangladesh but ethnic East Bengalis are darker and skinnier because they work in field under sunny environment all the time. Their skin became darker.
In todays Bangladesh it is moving from agricultural based economy to service based economy. So over the time Bangladeshis have become lighter than previous time.