以下是使用iText库遍历整个PDF文件，并将蓝色更改为黑色（同时更改下划线的颜色）的代码示例：

import com.itextpdf.kernel.colors.Color;
import com.itextpdf.kernel.colors.ColorConstants;
import com.itextpdf.kernel.pdf.*;
import com.itextpdf.kernel.pdf.canvas.parser.listener.ITextExtractionStrategy;
import com.itextpdf.kernel.pdf.canvas.parser.PdfTextExtractor;
import com.itextpdf.kernel.pdf.canvas.parser.listener.LocationTextExtractionStrategy;
import com.itextpdf.kernel.pdf.canvas.parser.listener.TextRenderInfo;
import com.itextpdf.kernel.pdf.canvas.parser.filter.TextRegionEventFilter;
import com.itextpdf.kernel.pdf.canvas.parser.listener.FilteredTextEventListener;

import java.io.IOException;

public class PDFColorChange {
    public static void main(String[] args) {
        String inputPath = "input.pdf";
        String outputPath = "output.pdf";

        try {
            PdfDocument pdfDoc = new PdfDocument(new PdfReader(inputPath), new PdfWriter(outputPath));
            for (int pageNum = 1; pageNum <= pdfDoc.getNumberOfPages(); pageNum++) {
                PdfPage page = pdfDoc.getPage(pageNum);
                PdfCanvas canvas = new PdfCanvas(page);

                FilteredTextEventListener listener = new FilteredTextEventListener(new LocationTextExtractionStrategy(), new TextRegionEventFilter(new Rectangle(0, 0, page.getPageSize().getWidth(), page.getPageSize().getHeight())));
                PdfCanvasProcessor parser = new PdfCanvasProcessor(listener);
                parser.processPageContent(page);

                for (TextRenderInfo renderInfo : listener.getResultantTextRenderInfos()) {
                    if (renderInfo.getFontColor() != null && renderInfo.getFontColor().equals(ColorConstants.BLUE)) {
                        canvas.setFillColor(ColorConstants.BLACK, true);
                        canvas.setStrokeColor(ColorConstants.BLACK);
                        canvas.setTextRenderingMode(PdfCanvasConstants.TextRenderingMode.FILL_AND_STROKE);
                    }

                    canvas.beginText();
                    canvas.setFontAndSize(renderInfo.getFont(), renderInfo.getFontSize());
                    canvas.setTextMatrix(renderInfo.getMatrix());
                    canvas.showText(renderInfo.getText());
                    canvas.endText();
                }
            }
            pdfDoc.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

请将input.pdf替换为要处理的PDF文件的路径，并将output.pdf替换为输出文件路径。此代码将遍历整个PDF文件，并将蓝色文本更改为黑色，并将下划线的颜色也更改为黑色。