Race, population and early voting overlap heavily in Ohio. In 2012, counties with higher densities of black residents were also the ones that cast more early votes. (The data below is compiled using data released by the Ohio Secretary of State at the time, accessed via the Internet Archive.)
Pulling out the eight counties that have 10 percent black population or higher -- mostly counties that contain the state's large cities -- the per-day early vote grew gradually over the course of the 2012 campaign. (The data below is an average based on the number of early votes cast during the period described.)
Over the course of the 2012 election, there were repeated skirmishes between the Republican secretary of state and advocates for black voters (who are far more likely to vote Democratic) regarding if and when early voting should be cut. In the end, the early voting was preserved, making it hard to estimate what the effect on turnout might have been had it not been.
Reducing a week of early voting this year, including nixing one of the last three days, could mean 200,000 fewer voters (the number who voted in the last full week in 2012) -- about 40 percent of whom come from those higher-density black counties. Or not. It's not clear how much people will adapt to the shifts in early voting schedules. Turnout in the state has remained relatively stable over the course of the last few election cycles. Early voting began in 2006, in response to the messy election day problems seen in the state, mostly in its larger cities. Since 2006, turnout hasn't moved around much. That stasis is despite early votes comprising over 10 percent of the vote total in 2012.
Compared to white turnout and to the national black turnout, black voters in Ohio vote at relatively high rates -- both before and after the early voting expansion. Black voters turned out at higher rates than white voters and black voters nationally in 2008 and 2012, for example.
Turnout is a game of small percentages, with dozens of voters in dozens of places adding up to thousands of additional votes. There's no question that some people who would have voted this year are less likely to now to do so with the scaling back of the early vote. But the number of variables at play and the up-and-down history in the state makes it almost impossible to predict with any accuracy exactly how much.