We know that from Pottermore there were 28 pure blood families in the 1930s. A 'Pure-Blood Directory' was subsequently published:
The so-called 'Sacred Twenty-Eight' comprised the families of:
Abbott, Avery, Black, Bulstrode, Burke, Carrow, Crouch, Fawley, Flint, Gaunt, Greengrass
Lestrange, Longbottom. Macmillan, Malfoy, Nott, Ollivander, Parkinson, Prewett, Rosier
Rowle, Selwyn, Shacklebolt, Shafiq, Slughorn, Travers, Weasley and Yaxley
Where did Tom Riddle/Voldemort fit in this list, and was he a pure-blood?