TRENTON, April 16---Two prominent young Trentonians aboard the Titanic are Washington A. Roebling, second, and Stephen W. Blackwell, who were returning home from an automobile trip through Europe.  Mr. Roebling is a son of Charles G. Roebling, president of the John A. Roebling’s Sons Company, and a nephew of Colonel Washington A. Roebling, of Brooklyn Bridge fame.Mr. Blackwell is the eldest son of former Senator Jonathan H. Blackwell and as graduated from Princeton in the class of ‘88.  The younger Mr. Blackwell had been in ill heath for several years, and it was thought that the trip abroad might benefit him.Apart from the passenger list, positive information that Mr. Roebling and Mr. Blackwell were aboard was contained in a cablegram announcing their departure when the steamer sailed.  Information regarding the wreck was sought eagerly yesterday and today by both the Roebling and Blackwell families.Charles G. Roebling had been drawn as a member of the United States Grand Jury, sworn in today.  He was excused from duty because of the accident, and confined himself to his home, where he received frequent bulletins from the New York agency of the Roebling company, giving all information available at the White Star offices.United States Senator Frank O. Briggs, an officer of the Roebling company, called upon Mr. Roebling shortly before noon.  All other visitors were informed that Mr. Roebling could not receive callers.About the only hopeful communication received today was a telephone communication, stating that a “Mr. Washington” was among the survivors.  It was thought that this might mean Washington A. Roebling, but the hope was a slender one.  Former Senator Blackwell spent most of yesterday at the White Star offices, retuning to Trenton last night, and informing his family of the extent of the catastrophe.  He went to New York again today in quest of further information as to his son.