Again, the $D is an argument and need to be in quotes.
echo $Dlets the shell do variable substitution then word splitting then globbing (replace wildcards with matching filenames). The word...
Yes, a consolidation makes sense.
MONTHYEAR=$(date +%b%Y)
DIR1=${MONTHYEAR}_Blast_BC01
DIR2=${MONTHYEAR}_Blast_BC15""Quoting of variables is never a mistake. Especially beginners should do it...