I dont understand. If the seal between the tiles and the tray has lifted from the tray, the water should be going in underneath the shower tray. What has the grout going black got to do with this problem ? Grout in showers generally after a while, does get dirty and needs cleaning (usually an old toothbrush is good for cleaning it off). You could try sealing the seal strip yourself by getting a tube of Silicon. You can buy them now ready squeezable (for want of a better word) without needing a silicon gun to do the job. Put a line of silicon along the tray where the seal strip should lie and press down for a minute or so till it sets.