Its the extra water for sure, unless your vascular smooth muscle is responding in weird way to the influx of androgens (vasoconstriction)
on 500 test my bp would be at 140/90 and i would be all red in my face.
on 250 test 100 tren a bp perfect, 120/70
also afaik test can cause water retention through other mechanisms other than high estro, think deca, it causes bloating while it aromatizes at a low rate
High bp really is common with aas like test, deca, npp...