The fuel that leaked out from the aircraft at times formed a visible puddle under the centre line of the aircraft. Ground crews were not too concerned as the flashpoint of the special grade fuel was extremly low.
The Blackbird had its own fleet of modified tankers, designated KC-135Q, that were able to carry the specific fuel that was required.
Another interesting point, the blackbird, tho painted black, actually changed to a dark blue colour at the top end of its flight envelope, this was again due to the extreme altitudes and speed.
The launch of a mission was usually done in complete radio silence. The pilot and control tower and chase car, used various communications techniques to get around the radio silence, this was to prevent 'big ears' from listening in. With nearly every mission, the aircraft was deployed over sensitive areas, the need for the silence was understandable.
In the end, i believe a hand held torch was used to give the pilot the all clear for departure.

While we're on some 'war' stories so to speak! I'd like to add that in Brian Shuls Sled Driver book, one of the many contributors added a true story about when they used a newly built engine run shed purposely built for the SR.

Unknowingly they closed the doors to the hangar and ran the SR up to full afterburner and the resulting suction started to drag equipment and personnel towards the back of the hangar!!!

Needless to say future engine runs were completed with the doors open!

And to add to Trevyuk2001s comment, they used to flash a light from the control tower to indicate clearance to takeoff! Obviously South Eastern Englands ATC and Radar controllers were aware of every missions start route up to 60,000 where the transponder was switched off by the crew and all external lights extinguished. The only people who knew where she went next were the planners, SAC, the mobile crew (who followed every flight from preflight to post touchdown and were SR71 crews) and obviously the mission crew.