Bludgeon is a short, heavy club which is thicker or loaded at one end.
Both OED and Etymonline say "origin unknown". There are possible Cornish, Celtic, Dutch, cant, Middle French, Irish and Gaelic origins suggested in various sources.
OED mentions the possible Cornish, Celtic, Dutch origins and the connection with blood:
Not found before the 18th cent.: origin unknown.
Blogon (with g = j) is quoted by Dr. Whitley Stokes from the Cornish drama Origo Mundi (? 14th cent.), but its relation to the English is uncertain. Other Celtic etymologies sometimes proposed are on many grounds untenable. A Dutch verb bludsen to bruise, has also been compared; and it has been suggested that the word is of cant origin, connected with blood.
In OED, the first citation is from a dictionary also:
1730 N. Bailey et al. Dictionarium Britannicum Bludgeon, an oaken stick or club.
Wiktionary mentions the Cornish and Middle French origins:
First attested in 1730. Origin uncertain, perhaps of Cornish origin (recorded as blogon c. 1450) or from Middle French bougeon, a diminutive of bouge (“club, stick”).
The Concise Dictionary of English Etymology (By Walter W. Skeat) mentions the Irish and Gaelic origins:
a cudgel. (C.) Irish blocan, dimin. of ploc, a block; Gael. plocan, a mallet, bludgeon, club, dimin. of ploc, a block; W. plocyn, the same.
Is it possible to trace back further and/or find a more definitive answer/source?