Errr, if Franco is worried about a British blockade, then joining the Axis - and completely cutting off all his trade with the Americas - is the last thing he wants to do.
On a balance of probabilities Franco did the best thing for Franco by keeping Spain neutral.
I reckon that if he had "done a Mussolini" in June 1940 (the only plausible time when he would have entered the war in my opinion because the war appeared to be over bar negotiating the peace treaty) it's 80:20 that the would have been dead by the end of 1945.
However, if (in common with his friend Benny) he miscalculated an did declare war on the WAllies an immediate British blockade of Spain might not be possible. The Royal Navy was overstretched as it was and might not be able to impose an effective blockade on Spain, at least in the short term.
The length of coastline to be covered would be longer. Furthermore Ferrol and Cadiz would be harder to blockade than the French Atlantic ports. Though it would help if Franco (unlike Mussolini) had ordered his merchant marine to head for home or neutral ports before declaring war.
And it might (10:90 chance) encourage friendly countries in Latin America to join the Axis. So instead of a butterfly effect, the domino effect comes into play.
It's not likely, but like the Channel Dash it's possible.