Little skip in webm videos joined with mkvmerge
Try a video editor like Lightworks to join them into one video, there you can cut any black frames and won't have to deal with possible glitches introduced by muxing different videos into one stream.

If your videos are indeed webm and not h264 you might need transcode them into another codec (f.e. with ffmpeg), a production codec like MJPEG or any lossless codec might be suitable as an intermediate codec so you don't loose much visual quality after exporting your finished video.