以下是使用iText库遍历整个PDF文件、将蓝色改为黑色并移除下划线的示例代码：

import com.itextpdf.io.font.FontConstants;
import com.itextpdf.kernel.colors.ColorConstants;
import com.itextpdf.kernel.font.PdfFont;
import com.itextpdf.kernel.font.PdfFontFactory;
import com.itextpdf.kernel.geom.Rectangle;
import com.itextpdf.kernel.pdf.*;
import com.itextpdf.kernel.pdf.canvas.parser.PdfCanvasProcessor;
import com.itextpdf.kernel.pdf.canvas.parser.listener.ITextExtractionStrategy;
import com.itextpdf.kernel.pdf.canvas.parser.listener.LocationTextExtractionStrategy;

import java.io.File;
import java.io.IOException;

public class RemoveUnderlineFromPDF {

    public static void main(String[] args) {
        try {
            // 加载PDF文件
            PdfReader reader = new PdfReader("input.pdf");
            PdfWriter writer = new PdfWriter("output.pdf");
            PdfDocument document = new PdfDocument(reader, writer);

            // 获取所有页面
            int numPages = document.getNumberOfPages();
            for (int pageNum = 1; pageNum <= numPages; pageNum++) {
                PdfPage page = document.getPage(pageNum);

                // 遍历页面内容
                PdfCanvasProcessor processor = new PdfCanvasProcessor(new MyRenderListener(page));
                processor.processPageContent(page);

                // 移除下划线
                for (Rectangle rect : processor.getRectangles()) {
                    PdfCanvas canvas = new PdfCanvas(page);
                    canvas.rectangle(rect);
                    canvas.setColor(ColorConstants.WHITE, true);
                    canvas.fill();
                }
            }

            document.close();
            System.out.println("PDF文件处理完成！");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static class MyRenderListener extends LocationTextExtractionStrategy {

        private PdfPage page;
        private PdfCanvas canvas;
        private PdfFont font;
        private Rectangle rect;

        public MyRenderListener(PdfPage page) throws IOException {
            this.page = page;
            this.canvas = new PdfCanvas(page);
            this.font = PdfFontFactory.createFont(FontConstants.HELVETICA);
        }

        @Override
        public void renderText(TextRenderInfo renderInfo) {
            String text = renderInfo.getText();
            if (text != null && !text.isEmpty()) {
                PdfString pdfString = new PdfString(text);
                if (pdfString.toUnicodeString().contains("http://") || pdfString.toUnicodeString().contains("https://")) {
                    rect = renderInfo.getDescentLine().getBoundingRectangle();
                    canvas.rectangle(rect);
                    canvas.setColor(ColorConstants.WHITE, true);
                    canvas.fill();
                } else if (renderInfo.getFillColor().equals(ColorConstants.BLUE)) {
                    Vector startPoint = renderInfo.getStartPoint();
                    Vector endPoint = renderInfo.getEndPoint();
                    canvas.saveState();
                    canvas.setColor(ColorConstants.BLACK, true);
                    canvas.setLineWidth(0.5f);
                    canvas.moveTo(startPoint.get(0), startPoint.get(1));
                    canvas.lineTo(endPoint.get(0), endPoint.get(1));
                    canvas.stroke();
                    canvas.restoreState();
                }
            }
            super.renderText(renderInfo);
        }

        public Rectangle getRect() {
            return rect;
        }
    }
}

请确保已将iText库添加到您的项目中。在示例代码中，我们首先加载输入PDF文件，然后遍历每个页面的内容。对于每个页面，我们使用PdfCanvasProcessor来遍历页面内容，查找包含“http//”和“https//”的文本，并将其下划线移除。对于蓝色文本，我们使用PdfCanvas来绘制黑色替代线条，以将其颜色改为黑色。

最后，我们将处理后的PDF文件保存为output.pdf。请将input.pdf替换为您要处理的实际PDF文件路径。