Not sure that I selected the proper table but here is the idea:
oTable.Cell(0, 0).Select(); //select the cell
//set up the left, right and top borders invisible (may be you don't need to do that)
oTable.Range.Borders[WdBorderType.wdBorderLeft].LineStyle = WdLineStyle.wdLineStyleNone;
oTable.Range.Borders[WdBorderType.wdBorderRight].LineStyle = WdLineStyle.wdLineStyleNone;
oTable.Range.Borders[WdBorderType.wdBorderTop].LineStyle = WdLineStyle.wdLineStyleNone;
//set up the bottom border blue
oTable.Range.Borders[WdBorderType.wdBorderBottom].LineStyle = WdLineStyle.wdLineStyleSingle;
oTable.Range.Borders[WdBorderType.wdBorderBottom].LineWidth = WdLineWidth.wdLineWidth050pt;
oTable.Range.Borders[WdBorderType.wdBorderBottom].Color = WdColor.wdColorBlue;
Usually if I want to do something and I don't know how to do that just open office program (Word in your case), start macro, do something, record and then see the generated code. Usually it would be enough to understand how to implement it.