Steve McQueen constantly smoked cigarettes during filming even though he is never shown smoking in the movie. (at around 49 mins) Smoke can be seen coming up behind McQueen. That's because he is holding a lit cigarette behind him with his hand behind his back.
The actual Blob, a mixture of red dye and silicone, has never dried out and is still kept in the original five-gallon pail in which it was shipped to the production company in 1958 from Union Carbide. It was put on display over the years as a part of the annual Blobfest, held over a three-day period each summer in Phoenixville, PA, which provided a number of the shooting locales for the film. In addition to displaying The Blob and miniatures used in the shooting, the event features a reenactment of the famous scene in which panicked theatergoers rush to exit the town's still-functioning Colonial Theater, as well as several showings of the film.
For the diner scene, a photograph of the building was put on a gyroscopically-operated table onto which cameras had been mounted. The table was shaken and the Blob rolled off. When the film negative was printed in reverse, it appeared to be oozing over the building.